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Jesús Murillo Karam, the attorney general, said a team of investigators from Mexico, Spain, the United States and Britain had found no evidence of explosives. He noted that there were no burn marks like those usually produced by explosives, nor were there signs of a crater, nor did investigators find any bomb-making materials in the office building where the blast occurred Thursday, just behind the company's Pemex tower.

"We found no residue of any kind of explosive device," Mr. Murillo said. He added that it had been a "diffuse" explosion, causing damage consistent with an accumulation of gas. The pressure pushed several floors of the building up, he said, and then they fell, collapsing on dozens of workers, including two more found dead this weekend buried in the rubble.

His explanation, delivered at a news conference late Monday, brought to a close several days of speculation. The government had been heavily criticized for not sharing enough information about the cause even as experts warned that investigations of this kind often take several days to figure out.

There are still some unanswered questions. Mr. Murillo said officials had yet to discover the source of the gas, which had built up in the basement of the building. Investigators believe it was methane that leaked from several ducts and tunnels underneath or connected to the building, he said. Why they leaked, who failed to notice (Pemex is responsible for inspecting its own buildings) and what exactly caused the gas to explode have not been clearly determined.

Mr. Murillo said that while there appeared to be no evidence of criminal wrongdoing, criminal charges were still a possibility.

When the blast occurred in the basement of an administrative building next to the 52-story tower, about 4 p.m. Thursday, windows shattered, the ground shook and thousands of panicked employees fled.

At the time, company officials said there was significant damage to the first floor and mezzanine of the building, and witnesses said they saw rescue workers helping trapped employees who had been pinned under falling debris, while others dragged out the injured and the dead.

The future of Pemex is a subject of debate. The national institution has been plagued by declining production, theft and an abysmal safety record that includes a major pipeline explosion almost every year. A pipeline blast in September killed 30 workers.

On Monday, the Knicks dismantled the Pistons for the third time this season, 99-85.

There were plenty of similarities between Monday's game and the one played in Britain in January, that one a 102-87 Knicks victory. Carmelo Anthony led all scorers with 27 points Monday, after posting 26 in London. The Knicks began Monday's game by building a 20-6 lead. They had a 16-2 lead after the first five minutes in London.

But Tyson Chandler provided one notable difference, giving the Garden crowd a reason to remain engaged in the fourth quarter Monday in a game whose outcome had long been Chandler was responsible for the biggest highlight — and the loudest roar from the crowd — midway through the fourth. After the Pistons' Will Bynum missed a free throw with 6 minutes 3 seconds left, Chandler corralled the ball to secure his 20th rebound, reaching that total for the third straight game. The last Knick to achieve the feat was Willis Reed in 1969. No N.B.A. player has recorded 20 rebounds in four straight games.

"It's a tough feat," Chandler said. "It hasn't been done in a long time since Willis Reed, and anytime you are mentioned in the same breath, it's a tall task."

Chandler had plenty of opportunities for rebounds against the Pistons, who shot 40 percent from the field and missed 52 shots. When Chandler came out of the game with 4:49 remaining he received a warm ovation.

The Knicks (31-15) have won five straight games, all at the Garden. That streak has allowed them to stay within a half-game of the Miami Heat for the top spot in the Eastern Conference. The Knicks now have a .673 winning percentage, compared with the .375 clip they were playing at a year ago.

After the Knicks left London, Woodson said he gave his team a goal over their remaining games before the All-Star break, which begins Feb. 14.

"I said we had to go 9-2 in that stretch and we're on pace if we continue to win," Woodson said before the game. "That's important because when we come back off this All-Star break, it really gets tough. Games are going to start coming so fast at us. March will be a tough month, so we have to put ourselves ahead of the pace going into the All-Star break so we're prepared for March when we come back."

The Knicks will play the Wizards in Washington on Wednesday, giving them a chance to extend their winning streak and offering Chandler an opportunity to do something not even Reed could do.

REBOUNDS

Monday's game marked the one-year anniversary Jeremy Lin's sudden rise with the Knicks last season, and the start of Linsanity. Lin, an undrafted free agent from Harvard, had 25 points, 7 assists and 5 rebounds in a victory against the Nets. Lin's last game in a Knicks uniform was against the Pistons on March 24. After that game, Lin, now with the Houston Rockets, rested his knee. He had season-ending surgery April 2.

Not that lilting instrumentals or mixes of pop hits are likely to disappear from restaurants, shops and offices. But the Muzak name — long part of the American vernacular, if sometimes as the butt of jokes — will be retired this week as part of a reorganization by its owner, Mood Media.

Mood Media, based in Concord, Ontario, has become a leader in so-called sensory marketing, providing stores and other businesses the sights, sounds and even smells to envelop their customers. In addition to Muzak, which it bought two years ago for $345 million, Mood has divisions for signs, interactive displays and scents, which it says reach 150 million people each day at more than 500,000 locations around the world, from Saks Fifth Avenue to Petco.

On Tuesday, the company will announce that it is consolidating its services under a single brand, Mood, thus eliminating the Muzak name.

"It's the end of an iconic American brand," said Lorne Abony, Mood Media's chairman and chief executive.

The move reflects the growing sophistication of in-store services, as well as the pressures facing physical retailers in the Internet age. At Mood's interactive kiosks, for example, shoppers can try on clothes virtually, while the company pipes in upbeat songs and scents intended to set a mood or cover up unpleasant odors.

"There's a huge opportunity and a need for physical retailers to make the experience more interactive as they do battle against online channels," said Edward S. Williams, a digital entertainment analyst at BMO Capital Markets in New York.

Mr. Abony said that Mood's background music services, including Muzak and DMX — which it bought last year — generate about 90 percent of the company's sales. In the third quarter of last year, it had $120 million in revenue and $32 million in earnings before interest, depreciation and amortization.

But the company wants to expand its other services by offering clients a broad menu under one name, particularly in the United States, where the penetration of Mood's sights-and-scents business has been low compared with the rest of the world.

"We have a team of music gurus, visual specialists, sound and scent-tech experts," Mr. Abony said. "We develop compelling, consistent experiences that connect our clients with their customers. The new brand signifies the integration of the company."

As part of Mood's incorporation of Muzak and DMX, the company has also reduced staff, although it has not said by how much.

Muzak traces its roots to the 1920s, but the brand name appeared in 1934. It evolved from simple background music in hotels and restaurants to a scientifically designed program to increase workers' productivity and make shoppers more comfortable.

Eventually the name came to be shorthand for any kind of innocuous musical wallpaper, even if in recent years Muzak and its competitors have also developed radiolike playlists of pop hits and oldies.

"Music by Muzak became a pervasive soundtrack, accompanying activities in offices, factories, supermarkets, hotel lobbies and even the Apollo 11 journey to the moon," said Joseph Lanza, the author of "Elevator Music: A Surreal History of Muzak, Easy-Listening, and Other Moodsong."

Although Mr. Abony cited the company's integration plans as the main reason for dropping the Muzak name, he also said that Mood had studied consumers' opinions about Muzak. Aside from the fact that it was little known outside the United States, he said, the brand had some baggage.

"It is often perceived as an epithet for elevator music," he said. "Muzak was not the connotation that suggested that we have come a long way."

Experts say a determination that Hezbollah was involved would force the European Union to reconsider whether to designate the group a terrorist organization, as the United States and Israel have urged.

The European calculation all along has been that whatever its activities in the Middle East, Hezbollah does not pose a threat on the Continent. Thousands of Hezbollah members and supporters operate in Europe essentially unrestricted, raising money that is funneled back to the group in Lebanon.

Changing the designation to a terrorist entity raises the prospect of unsettling questions for Europe — how to deal with those supporters, for example — and the sort of confrontation governments have sought to avoid.

"There's the overall fear if we're too noisy about this, Hezbollah might strike again, and it might not be Israeli tourists this time," said Sylke Tempel, editor in chief of the German foreign affairs magazine Internationale Politik.

The significance of their determination has put pressure on Bulgarian officials, who would like to maintain strong ties with both Israel and the United States, which call Hezbollah a terrorist organization, and European allies like France and Germany, which do not. Bulgarian officials have maintained a studied silence for more than six months since the attack.

"They have to name a name and say who is behind the attack," said Dimitar Bechev, head of the Sofia office of the European Council on Foreign Relations. "At the same time, they don't want to go solo. Whatever they say has implications not just for Bulgaria but for the E.U. as a whole."

A spokesman for the president's office, Veselin Ninov, said that the interior minister, Tsvetan Tsvetanov, was expected to "announce the results of the interim progress report in the investigation of the Burgas attack" on Tuesday, after the meeting of the president's council for national security, which includes the prime minister, top cabinet members and military and security personnel. Mr. Ninov would not say whether responsibility for those behind the bombing or their identities would be revealed.

Bulgarian officials are acutely aware of the stakes without anything like significant lobbying by larger European Union members that would prefer not to have to deal with the Hezbollah question. "It was not a campaign," said Philipp Missfelder, a leading member of Germany's Christian Democrats and the party's foreign-policy spokesman in Parliament. "Some German officials dropped a few words."

But Mr. Missfelder said that attitudes toward Hezbollah were gradually shifting. "It's clear that they are steered from Iran and they are destabilizing the region," Mr. Missfelder said. "The group that thinks Hezbollah is a stabilizing factor is getting smaller."

Hezbollah's dual nature as what Western intelligence agencies call a terrorist organization and a political party with significant social projects, including schools and health clinics, make it more difficult to dismiss. Hezbollah is a significant political actor in Lebanon, and many European officials are particularly wary of upsetting the status quo as the civil war drags on in Syria.

A sort of modus vivendi exists where Hezbollah keeps a low profile for its fund-raising and other activities and Europeans do not crack down. In Germany alone, some 950 people have been identified as being associated with the organization as of 2011. The group has always been treated as a benign force, even if assessments of the danger it presented varied greatly.

Omid Nouripour, a Bundestag member and a Green Party spokesman on security issues, said that for years he had opposed listing Hezbollah as a terrorist organization. "In the situation now, with Syria, I think it's now time to isolate Hezbollah," Mr. Nouripour said.

But France remains the European Union country with the strongest engagement in Lebanon as well as in Syria, and with the most say in European policy toward Hezbollah, said François Heisbourg, special adviser to the Foundation for Strategic Research in Paris.

"The French, to the extent that it's possible, try to avoid political destabilization and radicalization in Lebanon," Mr. Heisbourg said. "The driver in France is the situation in Lebanon and the politics in Lebanon. It's not as if France didn't know that Hezbollah is a terrorist organization."

The Netherlands has already declared Hezbollah a terrorist organization. Britain lists only the group's militant wing as a terrorist organization, distinguishing it from the political side. The United States and Israel have been the most vocal about the group's connection to violence and ties with Iran.

"I would describe Hezbollah as the most potent terrorist organization in the world, and very disciplined," said Michael Chertoff, the former secretary of homeland security under President George W. Bush.

A tour bus carrying Israeli vacationers exploded in a fireball outside the airport in Burgas, killing seven people including the bomber. Almost immediately the attack took on political overtones, with international echoes in Jerusalem, Beirut and Tehran.

Immediately after the bombing Prime Minister Benjamin Netanyahu of Israel blamed Iran and what officials there call its surrogate, Hezbollah, for the attack. Shortly afterward an American official, speaking on condition of anonymity, said that Tehran had given "broad guidance" to Hezbollah to attack Israeli targets when the opportunity presented itself. A senior government official in Israel said intelligence had identified telephone calls between Lebanon and Burgas in the two months leading up to the bombing, with a spike in the three days before the attack.

The bombing, which came in the wake of several near-misses and foiled plots against Israeli interests around the world, was widely seen as retribution for the killings of Iranian nuclear scientists, part of what analysts have called a shadow war pitting Iran and Hezbollah against Israel. Iran blames Israel for the assassinations of several nuclear scientists. Israel says Hezbollah and Iran are behind a series of attacks on Israeli targets in India, Georgia, Thailand and elsewhere, in addition to the Burgas bombing.

Shortly after the attack Prime Minister Boiko Borisov of Bulgaria called the group behind it "an exceptionally experienced" team, and described the attack as a conspiracy with multiple members, not the act of an individual. A member of the country's security establishment said that there was a "clear direction that points to Hezbollah" in both the pattern of the attack and the evidence.

But the Bulgarians stopped short of formally blaming Hezbollah at the time. Six months have passed without a determination by Bulgarian officials.

"If you factor in the suspicion that there are political implications beyond Bulgaria's borders," said the European council's Mr. Bechev, "it's completely understandable that they've been playing for time."

But it was not the helicopters hovering overhead or the antiaircraft weapons or the soldiers in camouflage that caught their attention. It was the man chosen to march at the head of the parade, Gen. Moisés García Ochoa, who by tradition typically becomes the country's next minister of defense.

The Obama administration had many concerns about the general, from the Drug Enforcement Administration's suspicion that he had links to drug traffickers to the Pentagon's anxiety that he had misused military supplies and skimmed money from multimillion-dollar defense contracts.

In the days leading up to Mexico's presidential inauguration on Dec. 1, the United States ambassador to Mexico, Anthony Wayne, met with senior aides to President Enrique Peña Nieto to express alarm at the general's possible promotion.

That back-channel communication provides a rare glimpse into the United States government's deep involvement in Mexican security affairs — especially as Washington sizes up Mr. Peña Nieto, who is just two months into a six-year term. The American role in a Mexican cabinet pick also highlights the tensions and mistrust between the governments despite public proclamations of cooperation and friendship.

"When it comes to Mexico, you have to accept that you're going to dance with the devil," said a former senior D.E.A. official, who requested anonymity because he works in the private sector in Mexico. "You can't just fold your cards and go home because you can't find people you completely trust. You play with the cards you're dealt."

A former senior Mexican intelligence official expressed similar misgivings about American officials. "The running complaint on the Mexican side is that the relationship with the United States is unequal and unbalanced," said the former official, who, like others interviewed for this article, spoke anonymously to discuss diplomatic and security exchanges. "Mexico is open with its secrets. The United States is not. So there's a lot of resentment. And there's always an incentive to try to stick it to the Americans."

Wave of Violence

Washington's concerns about General García Ochoa — which several officials cautioned were not confirmed — come as both governments grasp for new ways to stem the illegal flows of drugs, guns and money across their borders.

Under Mr. Peña Nieto's predecessor, Felipe Calderón, cooperation between the two governments had expanded in ways once considered unthinkable, with American and Mexican agents conducting coordinated operations that resulted in the capture or killing of several dozen important cartel leaders. But while Washington highlighted the record numbers of arrests, the stepped-up campaign created a wave of violence in Mexico that left some 60,000 people dead.

The devastating death toll has Mr. Peña Nieto, 46, a former governor, promising to move his country's fight against organized crime in a different direction, focusing more on reducing violence than on detaining drug kingpins. But he has so far offered only vague details of his security plans, focusing instead on social and economic programs.

While Mr. Peña Nieto portrays himself as the leader of a new generation of reformers, he is also a scion of the Institutional Revolutionary Party, or PRI, which ruled Mexico for more than 70 years through a combination of corruption and coercion until it lost power in 2000. During its time in power, the party was known more for keeping the United States at arm's length while attempting to strike deals with drug traffickers, rather than combating them head on.

Mr. Peña Nieto's election has brought the PRI back to power, and since so many of those serving in his cabinet have one foot in the past, foreign policy experts who specialize in Mexico say it is not clear where the new government is headed.

"It could go either way," said Eric L. Olson of the Woodrow Wilson International Center for Scholars in Washington, speaking of future cooperation between Mexico and the United States. "Part of me says, 'Let's not assume it's all going to go south.' And there are things that are happening that give me hope. But the longer it goes without some clarity, the more doubts creep in."

Those doubts have also crept to Capitol Hill. Senator Patrick J. Leahy, a Democrat from Vermont who sits on the Senate Appropriations Committee, said he was withholding nearly $230 million in security assistance to Mexico through the so-called Merida Initiative amid concerns about whether the fight against organized crime is doing more harm than good.

"Congress has been asked for a significant new investment, but it's not clear what the Mexican government's plans are," Mr. Leahy said. "It's premature to sign off on more of the same."

General García Ochoa, 61, whose background is at once exemplary and enigmatic, personifies that quandary. On paper, he is a model officer. He earned two advanced degrees from Mexico's most prestigious military academies, and founded the elite National Center for Counter-Narcotics Intelligence. He has been a student and an instructor in American military training programs. He has written three books, including one on the military's role in the drug fight.

People who know the general said they were struck by his candid assessments of the fight against organized crime. He spoke openly about governmental corruption, a topic that has been considered taboo. And on at least two occasions over the past year and a half, the general's friends said, he traveled secretly to San Antonio to meet with American intelligence officials — he didn't feel safe meeting with agents in Mexico, they said — and gave names of military and civilian officials he suspected of providing protection to drug traffickers.

"He was genuinely worried that corruption was giving the military a bad name, and that if nothing was done about it, it could hurt relations with the United States," said a person knowledgeable about the meetings. "The way he saw it, this next government has the chance to really change the way Mexico works with the United States. He didn't want that chance to be missed."

By then, General García Ochoa was already on the short list to become defense minister. And people who know him said he hoped American support would give him an advantage over other candidates.

What he did not know was that the United States was quietly advocating against him. Current and former American officials said they had put together a troubling portfolio of allegations against the general. In his role as director of military administration and acquisitions, he had been accused of skimming money and supplies from large defense contracts.

Reports in the Mexican news media last summer accused the general of approving payments totaling more than $355 million for sophisticated surveillance equipment, without reporting those payments to civilian authorities or providing an explanation of how that equipment would be used.

'Mr. Ten Percent'

Behind the scenes, American officials had nicknamed the general "Mr. Ten Percent," shorthand for their suspicions about the way he handled contracts. And two American officials recalled the general making a formal request for American assistance for the military's helicopter unit, and then backing out of the arrangement when the United States asked to look at the books — including the unit's financial, flight and fuel records.

"The United States is sending a lot of money down there," said one senior American official, describing the concerns about the general. "We need to be sure that money is being used in the right way or we could lose a huge opportunity."

The D.E.A. suspected the general had long ties to drug traffickers. Agents declined to discuss the specific nature of those links. Nor would they say whether their investigation against the general was continuing. General García Ochoa declined requests to be interviewed.

"There was a lot of information on him, and it was coming from multiple sources," said a recently retired senior federal law-enforcement officer, referring to what he called the "serious concerns" about the general. "We never found any smoking guns, not enough to make a case."

The New York Times obtained classified D.E.A. intelligence reports from the early years of the general's career, when he founded the counternarcotics intelligence center. The reports, dated Dec. 15, 1997, allege that then-Colonel García Ochoa was one of several senior Mexican military officials involved with attempts to negotiate a deal with the country's most powerful drug trafficking organizations.

"It is highly likely," said one report, "that military officials wanted to continue to profit from an ongoing relationship with the drug traffickers."

The reports also allege that the colonel led a raid against the Juárez Cartel in which he deliberately allowed the kingpin Amado Carrillo Fuentes to escape, saying that the colonel "did not give orders to launch the operation until the car in which ACF was reportedly traveling had departed the area."

Mexican officials declined requests to be interviewed for this article. American officials declined to comment publicly on their suspicions about the general. But they emphasized that whatever concerns they might have had about an individual general were hardly representative of the larger relationship between the two governments.

There have been significant strides in cooperation in recent years, including the first drones flying over Mexican airspace, the creation of the first joint intelligence center on a Mexican military base, operations staged by Mexican counternarcotics officers on the United States side of the border, and operations conducted by American federal law enforcement agents against money laundering in Mexico.

The United States has successfully shared delicate intelligence with the Mexican Navy, which led to the arrests of significant cartel leaders. And the number of exchanges between the Pentagon and the Mexican military has increased drastically, from 3 events in 2009 to nearly 100 last year, according to a report in Small Wars Journal, an independent online military publication.

"One of the most important bilateral relationships the United States has is with Mexico, and neither side is going to abandon it," said another former senior D.E.A. official. "Yes, there are significant concerns, but when they come up you try to isolate them, limit their impact and move on."

The American effort to prevent General García Ochoa's promotion was just such an exercise in containment, with the Americans quietly moving to weed out Mexican officials suspected of corruption because they feared Mexican institutions would not be willing or able to do so on their own.

Misgivings Aired

After September's Independence Day parade, senior American officials gathered in Mexico City for two days of meetings to assess their suspicions about the general, and to discuss whether or not to share those misgivings with their Mexican counterparts.

According to a Mexican official, the Americans eventually did share their concerns about the general, less than a week before Mr. Peña Nieto announced his cabinet appointments. The official said the American ambassador met in Mexico City with two senior aides to the incoming leader, including Miguel Ángel Osorio Chong, who later became interior minister, and Jorge Ramírez Marin, a former national security adviser.

The official said Mr. Wayne, the ambassador, had discussed Washington's concerns about the general, emphasizing that the allegations had not been corroborated.

"The timing was important," the Mexican official said, "because Mexican presidents almost never replace the person they appoint as defense minister, so whoever was chosen would be involved with setting the terms of cooperation for the next six years."

In the end, General García Ochoa did not get the job. Instead, it went to Gen. Salvador Cienfuegos Zepeda, who Mexican officials said had become close with Mr. Peña Nieto when he served as governor of the state of Mexico and General Cienfuegos commanded the area's military base.

As for General García Ochoa, he was dispatched to a military base in the northern border state of Coahuila, a hotbed of cartel-related prison breaks, police corruption and political assassinations.

Whether Washington played a central role in how things turned out for the general remains unclear. Meanwhile, a column in the Mexican newspaper El Universal debated whether his dangerous new assignment was a demonstration of the government's confidence in him, or a demotion aimed at forcing him to consider an early retirement.

Whichever the case, the general made a hasty departure from the military's headquarters in Mexico City. One person who knows him said he had emptied his office with the help of a handful of aides and dispensed with the usual farewell festivities.

On a day in December when defense ministers from across the hemisphere gathered for a summit meeting in Mexico City, the general was seen wearing civilian clothes, climbing into his personal car and driving away.