Weapons-tracking organization Conflict Armament Research worked alongside Kurdish forces to recover more than 1,700 cartridges used by ISIS. More than 20 percent of the ammunition was U.S.-made.
"[ISIS] appear to have acquired a large part of their current arsenal from stocks seized from, or abandoned by, Iraqi defense and security forces," the report said.
Most of the U.S. ammo was manufactured in the first decade of the 21st century, when Washington was bolstering Iraqi forces following the fall of Saddam Hussein.
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The ammo recovered in the study came from 21 different countries and dated from 1945 to present day. The biggest contributors were China and the Soviet Union, with most of their ammo originating in the 1970s and 1980s. The study also found modern Iranian ammunition.
"If transferred deliberately, the presence of this ammunition outside of Iran violates U.N. Security Council Resolution 1737 (2006), which prohibits Iran's export of ammunition," it noted.
Investigators found this expended ammunition at ISIS firing positions in northern Iraq. The bullets were manufactured at the Lake City Army Ammunition Plant in Independence, Missouri.