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Within two years other researchers were finding identical cartridges circulating through the ethnic violence in Darfur. Similar ammunition then turned up in 2009 in a stadium in Conakry, Guinea, where soldiers had fired on antigovernment protesters, killing more than 150.

For six years, a group of independent arms-trafficking researchers worked to pin down the source of the mystery cartridges. Exchanging information from four continents, they concluded that someone had been quietly funneling rifle and machine-gun ammunition into regions of protracted conflict, and had managed to elude exposure for years. Their only goal was to solve the mystery, not implicate any specific nation.

When the investigators' breakthrough came, it carried a surprise. The manufacturer was not one of Africa's usual suspects. It was Iran.

Iran has a well-developed military manufacturing sector, but has not exported its weapons in quantities rivaling those of the heavyweights in the global arms trade, including the United States, Russia, China and several European states. But its export choices in this case were significant. While small-arms ammunition attracts less attention than strategic weapons or arms that have drawn international condemnation, like land mines and cluster bombs, it is a basic ingredient of organized violence, and involved each year and at each war in uncountable deaths and crimes.

And for the past several years, even as Iran faced intensive foreign scrutiny over its nuclear program and for supporting proxies across the Middle East, its state-manufactured ammunition was distributed through secretive networks to a long list of combatants, including in regions under United Nations arms embargoes.

The trail of evidence uncovered by the investigation found Iranian cartridges in the possession of rebels in Ivory Coast, federal troops in the Democratic Republic of Congo, the Taliban in Afghanistan and groups affiliated with Al Qaeda in the Maghreb in Niger. The ammunition was linked to spectacular examples of state-sponsored violence and armed groups connected to terrorism — all without drawing wide attention or leading back to its manufacturer.

The ammunition, matched to the world's most abundant firearms, has principally been documented in Africa, where the researchers concluded that untold quantities have been supplied to governments in Guinea, Kenya, Ivory Coast and, the evidence suggests, Sudan.

From there, it traveled to many of the continent's most volatile locales, becoming an instrument of violence in some of Africa's ugliest wars and for brutal regimes. And while the wide redistribution within Africa may be the work of African governments, the same ammunition has also been found elsewhere, including in an insurgent arms cache in Iraq and on a ship intercepted as it headed for the Gaza Strip.

Iran's role in providing arms to allies and to those who fight its enemies has long been broadly understood. Some of these practices were most recently reported in the transfer of Fajr-5 ground-to-ground rockets to Gaza. Its expanding footprint of small-arms ammunition exports has pushed questions about its roles in a shadowy ammunition trade high onto the list of research priorities for trafficking investigators.

"If you had asked me not too long ago what Iran's role in small-arms ammunition trafficking to Africa had been, I would have said, 'Not much,' " said James Bevan, a former United Nations investigator who since 2011 has been director of Conflict Armament Research, a private firm registered in England that identifies and tracks conventional weapons. "Our understanding of that is changing."

The independent investigation also demonstrated the relative ease with which weapons and munitions flow about the world, a characteristic of the arms trade that might partially explain how Iran sidestepped scrutiny of governments and international organizations, including the United Nations, that have tried to restrict its banking transactions and arms sales.

The United Nations, in a series of resolutions, has similarly tried to block arms transfers into Ivory Coast, Congo and Sudan — all places where researchers found Iranian ammunition.

Ammunition from other sources, including China, Russia, Hungary, the Czech Republic and other former Soviet bloc states remain in circulation in Africa, along with production by African states. Why Iran has entered the market is not clear. Profit motives as well as an effort by Iran to gain influence in Africa might explain the exports, Mr. Bevan said. But much remains unknown.

Neither the government of Iran nor its military manufacturing conglomerate, the Defense Industries Organization, or DIO, replied to written queries submitted for this article.

The researchers involved in the investigation — including several former experts for the United Nations and one from Amnesty International — documented the expanding circulation of Iranian ammunition, not the means or the entities that have actually exported the stocks. They are not sure if the ammunition had been directly sold by the Iranian government or its security services, by a government- or military-controlled firm, or by front companies abroad.

But the long mysterious provenance of the ammunition appears beyond dispute. The cartridges were made, the researchers say, by the Ammunition and Metallurgy Industries Group, a subsidiary of DIO.

Matching the cartridges to the producer took time, in part because the ammunition had been packaged and marked in ways to dissuade tracing. Eventually the identifications were reached via data-pooling.

Much of the world's ammunitions bears numeric or logo markings, known as headstamps, that together declare the location and year of a cartridge's manufacture. Over the years, governments and private researchers have assembled encyclopedic headstamp keys that can make matching particular markings to particular factories a straightforward pursuit.

The ammunition in these cases included rounds for Kalashnikov assault rifles, for medium machine guns and sniper rifles and for heavy machine guns.

In each case, the cartridges carried headstamps not listed on the publicly available records. The stamps were simple caliber markings and, typically, two digits indicating the year of manufacture.

Similarly, neither the ammunition's wooden crates nor its packaging in green plastic carry bags or plain cardboard boxes, when these items were found with the ammunition, disclosed the place of manufacture.

All of the ammunition shared a unique combination of traits, including the caliber headstamp in a particular font, the alloy of the bullet jackets, and three indentations where primers attached to cartridge cases. The traits together suggested a common manufacturer.

Over the years, the researchers bided time and gathered data. They collected samples of used and unused ammunition at conflicts and recorded their characteristics. They collaborated with other specialists, exchanging their finds.

Some sources were confidential. Others were not. Mike Lewis, a former member of the United Nations Panel of Experts on the Sudan, documented the presence of the ammunition at the Conakry stadium crackdown while investigating for Amnesty International.

One sample — from Afghanistan — was found by The New York Times, which was surveying ammunition used by the Taliban and provided an image of a then-unidentifiable cartridge's headstamp to Mr. Bevan in 2010.

Once the data was assembled, the breakthrough came in what a soon-to-be-released report by the researchers called "cross-case analysis" and by looking away from the ammunition to other sources.

In late 2011 Mr. Bevan obtained the bill of lading for 13 shipping containers seized by the authorities in Lagos, Nigeria, in 2010. The document showed that the containers originated in Iran and declared the contents to be "building materials."

But, as the researchers noted in their report, "concealed behind stone slabs and insulation materials" was a shipment of arms, including the same ammunition that they had been finding in the field.

The shipping company was based in Tehran, Iran's capital.

Declassified documents obtained via the Freedom of Information Act by Matthew Schroeder, an arms-trafficking analyst at the Federation of American Scientists, later showed that the American military had identified ammunition packaged in the same materials as Iranian. Mr. Schroeder shared his documents with Mr. Bevan. This provided another link.

Ultimately, Mr. Bevan noticed that Iran had published limited technical details of its cartridges, including bullet weights. Some of these weights are atypical. Late in 2012 he had samples weighed on a jeweler's scale, confirming the match.

Mr. Bevan made clear in repeated interviews that he and his fellow researchers are not advocates for military action against Iran. When they began tracing the ammunition, they did not know or expect that the evidence would point to Tehran.

He also noted that while the ammunition is Iranian-made, it may not have been sent to directly by Iran to some of the combatants.

"In terms of prescription, if it was clear that there were repeated violations by Iran, I think we could come down more strongly about it," he said. "But a good portion of this, and in perhaps the majority of these cases, the ammunition was transferred around Africa by African states."

He added that while the original source of the ammunitions is now clear, many questions remain unanswered, including who organized the delivery to regions under embargo or enmeshed in ethnic conflicts.

Mr. Bevan and his fellow researchers said their findings pointed to a need for further research, to gather facts upon which policy decisions can be based.