The Pentagon is investigating the fatal shooting of a wounded enemy fighter in Iraq by a U.S. Marine that was captured on videotape.

The shooting happened during fighting in Fallujah (search) at a mosque between insurgents and Marines (search) from the 3rd Battalion, 1st Marine Regiment, according to unidentified embedded reporters and the Pentagon.

The Marines from that regiment came under fire by rebel forces stationed at the mosque on Friday, and U.S. forces responded by launching machine-gun and tank strikes, according to reports from the field. Ten insurgents were killed and five wounded in the fighting, FOX News has learned.

The following day, another set of Marines from the same unit engaged in a battle at the same mosque after observing movement inside. An embedded correspondent reported seeing fresh gunshot wounds on those injured the previous day.

During the second-day operation, one of the Marines came upon a wounded insurgent lying on the floor. On the tape, Marines wielding machine guns are seen going into the mosque and approaching a couple of Iraqi fighters lying on the floor. One of the Marines is heard repeatedly shouting, “He’s faking he’s dead!”

The Marine then apparently fired a single shot into the wounded man’s head — which was heard and caught on videotape but blacked out when the tape was aired. A voice can be heard saying, “He’s dead now."

Shooting an unarmed, wounded combatant is considered a violation of the rules of war.

Pentagon sources told FOX News that the Marine who shot the wounded Iraqi has been taken off the battlefield and the incident is under investigation. It could have serious implications for the U.S. Marines and their conduct on the battlefield.

But military sources also told FOX News that enemy fighters have used the tactic of feigning a wound to draw U.S. soldiers in, then opening fire on them. The Marine who apparently killed the injured fighter had reportedly been shot in the face the day before, FOX News has learned.

Insurgents have also been known to rig dead bodies with explosive devices that go off when U.S. troops approach.

FOX News' Greg Kelly and Catherine Donaldson-Evans contributed to this report.