Investigators are looking into the possibility that a Marine Raider was killed by friendly fire in Iraq, U.S. military officials told Task & Purpose on Monday.

Gunnery Sgt. Scott A. Koppenhafer died on Aug. 10 of “injuries sustained during combat action in Iraq,” according to Marine Forces Special Operations Command.

The Pentagon's initial news release, which did not name Koppenhafer, said he had been killed, “After being engaged by enemy small arms fire while conducting combat operations.”

As part of the investigation into his death, U.S. military officials will look into whether Koppenhafer might have been accidentally killed by U.S. or Iraqi forces, Nancy Youssef of the Wall Street Journal first reported on Monday.

Much about the circumstances surrounding Koppenhafer's death remain unclear. U.S. officials stressed that the investigation will look into all possible causes of death, not just friendly fire.

Roughly 5,200 U.S. troops are currently serving in Iraq as part of the campaign against ISIS, said Army Lt. Col. Earl Brown, a spokesman for U.S. Central Command.

Koppenhafer is the third U.S. service member to die in Iraq this year. Two other U.S. troops have been died of non-combat injuries: Army Spc. Michael T. Osorio died on April 23 in Taji; and Army Spc. Ryan Dennis Orin Riley died on April 20 in Ninawa province.

In Syria four Americans killed by an ISIS suicide bomber in January: Army Chief Warrant Officer 2 Jonathan R. Farmer, Navy Chief Cryptologic Technician (Interpretive) Shannon M. Kent, Defense Department civilian Scott A. Wirtz, and Defense Department contractor Ghadir Taher.