The dramatic footage was taken on 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 marines 3rd battalion, 1st regiment, returned to the unidentified Falluja mosque on Saturday. Sites was embedded with the unit.
Sites 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, 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 on Friday with their drive to retake the city from armed fighters who have been battling US-led occupation forces in Iraq with increasing ferocity and violence in recent months.
The same five men were still in the mosque on Saturday, Sites reported.
Firing point blank
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!"
The footage shows a US marine
shooting at a wounded fighter
"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 towards a prisoner lying on the floor of the mosque.
The video shown by NBC and provided to the network pool was blacked out at that point and did not show the bullet hitting the man; a rifle shot could be heard.
However, Aljazeera and several other news media broadcast the footage without obstruction.
"He's dead now," a marine is heard saying.
The blacked out portion of the video, provided later to Associated Press Television News and other members of the network pool, shows 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.
Sites reported a marine in the same unit had been killed just a day earlier when he tended to the booby-trapped dead body of an anti-US fighter.
NBC reported that the marine seen shooting the wounded Iraqi had himself been shot in the face the day before, but quickly returned to duty.
A spokesman at Marine Corps headquarters in the Pentagon, Major Doug Powell, said the incident was "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 US military said in a statement that the 1st Marine Division was investigating an allegation of the unlawful use of force in the death of an enemy combatant in Falluja during combat operations on Saturday.
Three other wounded Fallujans
were shot a second time
The marine has been withdrawn from the battlefield pending the results of the investigation, the US military said.
"We follow the law of armed conflict and hold ourselves to a high standard of accountability," said Lieutenant General John Sattler, commanding general of the 1st Marine Expeditionary Force.
"The facts of this case will be thoroughly pursued to make an informed decision and to protect the rights of all persons involved."
Rules of engagement
Lieutenant Colonel Bob Miller, who NBC said is heading the investigation, told the network that the rules of engagement allow the use of force in self-defence.
"Any wounded - in this case insurgents - who don't pose a threat would not be considered hostile," said Miller.
The events on the videotape began as some of the marines from the unit accompanied by Sites approached the mosque on Saturday, a day after it was stormed by other marines.
Gunfire can be heard from inside the mosque, and at its entrance, marines who were already in the building emerge.
They are asked by an approaching marine lieutenant if there were any anti-US fighters inside and if the marines had shot any of them. A marine can be heard responding affirmatively. The lieutenant then asked if they were armed and a fellow marine shrugs.
Bled to death
Sites' account said the wounded men, who he said were prisoners and who were hurt in the previous day's attack, had been shot again by the marines on the Saturday visit.
The video showed two of the wounded men propped against the wall and Sites said they were bleeding to death. According to his report, a third wounded man appeared already dead, while a fourth was severely wounded but breathing.
The fifth was covered by a blanket, but did not appear to have been shot again after the marines returned. It was the fourth man who was shown being shot.
The CNN broadcast of the pictures used pixilation to cover parts of the video that could lead to public identification of the marines involved.
NBC's Robert Padavick told members of the US television pool that the Pentagon had ordered NBC and other pool members to make sure the marine's identity was hidden because "they [the military authorities] are anticipating a criminal investigation as a result of this incident and do not want to implicate anybody ahead of that".
In New York, NBC spokeswoman Allison Gollust said the network did not broadcast the prisoner being shot because of the graphic nature of the video.