The U.S. military is investigating whether friendly fire resulted in the death of a Marine in Iraq over the weekend, officials said.

The death of Marine Gunnery Sgt. Scott A. Koppenhafer represented the first U.S. combat casualty since troops returned to the country in 2014 to battle the Islamic State, The Wall Street Journal reported.

Koppenhafer, 35, of Colorado, was assigned to the 2nd Marine Raider Battalion, based in Camp Lejeune, N.C. He was part of a team comprising American and Iraqi security forces. It was conducting an operation Saturday in Nineveh Province.

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He was first reported to have died by small-arms enemy fire. The Pentagon is now looking into whether he was accidentally shot by U.S. or Iraqi forces instead.

Most of the 5,200 U.S. troops in Iraq train Iraqi security forces and conduct counterterrorism operations, according to the newspaper. A report by the Pentagon’s Office of Inspector General released earlier this year said ISIS militants were waging an "effective clandestine" insurgency in Iraq and Syria and still pose a significant threat.

President Trump announced a pullout of U.S. troops from Syria last year after asserting that ISIS had been defeated.

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The report said withdrawing American forces from both countries could "allow terrorists to regroup, destabilize critical regions and create vacuums that could be filled by Iran or Russia."

The joint American-Iraqi task force was established in 2014 to combat gains made by ISIS in Iraq after taking control of Mosul, the provincial capital.