The dramatic footage was taken Saturday by pool correspondent Kevin Sites of NBC television, who said three other prisoners wounded a day earlier in the mosque had also apparently been shot the next day by the marines.

The incident played out as the marine 3rd Battalion, 1st Regiment, returned to the unidentified Fallujah mosque Saturday. Sites was embedded with the unit.

He reported that a different marine unit had come under fire from the mosque on Friday. Those marines stormed the building, killing 10 men and wounding five, Mr. Sites said. The marines said the fighters in the mosque had been armed with rocket-propelled grenades and AK-47 rifles.

The marines had treated the wounded, he reported, left them behind and continued Friday with their drive to retake the city from guerrillas who have been battling U.S.-led occupation forces in Iraq with increasing ferocity and violence in recent months.

The same five men were still in the mosque Saturday, Mr. Sites reported.

On the video, as the camera moved into the mosque during the Saturday incident, a marine can be heard shouting obscenities in the background, yelling that one of the men was only pretending to be dead.

“He's (expletive) faking he's dead!”

“Yeah, he's breathing,” another marine is heard saying.

“He's faking he's (expletive) dead!” the first marine says.

The video then showed a marine raising his rifle toward a prisoner lying on the floor of the mosque. The video shown by NBC and provided to the network pool was stopped and did not show the bullet hitting the man, but the audio continued and a rifle shot could be heard.

“He's dead now,” a marine is heard saying.

The blacked out portion of the videotape, provided later to Associated Press Television News and other members of the network pool, showed the bullet striking the man in the upper body, possibly the head. His blood splatters on the wall behind him and his body goes limp.

Mr. Sites reported that a marine in the same unit had been killed just a day earlier when he tended to the booby-trapped dead body of an insurgent.

A spokesman at U.S. Marine Corps headquarters in the Pentagon, Major Doug Powell, said the incident is “being investigated.” He had no further details, other than to confirm the incident happened on Saturday and that the marines involved were part of the 1st Marine Division.

On Tuesday, the U.S. military said in a statement that the 1st Marine Division is investigating an allegation of the unlawful use of force in the death of an enemy combatant in Fallujah during combat operations on Saturday.

The marine has been withdrawn from the battlefield pending the results of the investigation, the U.S. military said.