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The cause of the explosion was not immediately known. It occurred just before 4 p.m. in the basement of an administrative building next to the 52-story tower of Petroleos Mexicanos, or Pemex. 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.

"I saw them take out three people covered in blood," said Trinidad Díaz, 31, the owner of a restaurant a block away from the explosion. "And after that, ambulances started arriving, one after the other."

The blast — in a highly protected but decaying office complex — comes in the middle of a heated debate over the future of Pemex, a national institution and a corporate behemoth that has been plagued by declining production, theft and an abysmal safety record that includes a major pipeline explosion almost every year, like the one in September that killed 30 workers.

Experts, while cautioning that it was too early to tell what had gone wrong, said the company would inevitably face more severe scrutiny as Mexico's Congress returned to work in the coming weeks. The country's new president, Enrique Peña Nieto, has pledged to submit a plan for reforming Pemex, opening it to more private investment and perhaps greater consolidation. But with the blast, deliberations about the company could become more elemental.

"You pull all of this together and you say, well, if they can't even guarantee safety in their own building, their own headquarters, what does that tell us about the company?" said Duncan Wood, director of the Mexico Institute at the Woodrow Wilson International Center for Scholars. "It tells us there are things seriously wrong there. It tells you things need to be seriously shaken up."

George Baker, director of Energia.com, an energy research institute in Houston, said that previous safety scandals at Pemex had been used by Mexican leaders as an argument for making controversial changes. In 1992, he said, a major explosion in a residential Guadalajara neighborhood — caused by gas leaking into the sewers — was followed by calls for change, and a plan to break Pemex into smaller pieces.

"The provocation, the pretext was that we had this terrible thing happen and now we are going to have a response from Pemex," Mr. Baker said, adding that the explosion on Thursday would also now become part of the political calculations over what to do about the company.

"This may be used, may be manipulated, used as a pretext to do something," he said. "Who knows what that something is, but they may exploit it to do something they were going to do anyway."

At the scene, employees who were visibly shaken said the explosion felt like a bomb or an earthquake. After a deep rumble, a plume of smoke rose skyward and people rushed into the streets. Four rescue helicopters landed in the area to remove the dead or injured, while a half-dozen more helicopters hovered overhead. Soldiers, police officers and ambulances filled the area, and streets were quickly cordoned off.

A team of three emergency responders who had entered the building soon after the blast said that it looked as if two basement floors and parts of three upper floors had collapsed. Papers were strewed everywhere, and the scent of dust lingered in the air. The those in the rescue team said another rescue worker who had gone inside told them he saw eight lifeless bodies.

Just before dark, local news outlets reported that President Peña Nieto had arrived. He had already demanded an investigation and expressed remorse, using his Twitter account. "I profoundly lament the death of our fellow workers at Pemex," he said on Twitter just before arriving. "My condolences to their families."

Pemex officials, using the company's official Twitter account to confirm that at least 14 people had died, said around the time of the explosion that its offices were being evacuated because of an electrical problem. Later, the company said forensic teams were investigating the cause, which had not been determined. "Any other explanation with respect to this is speculation."