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 countries. 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 is 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 included 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 Islamic 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 had 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.