The video showed a wounded insurgent on the floor of the room
The US military has announced it is looking into whether an American marine in Falluja shot dead a severely wounded Iraqi insurgent at point-blank range.
Television footage shows US soldiers entering a building as injured prisoners lie on the floor.
The soldier, who has not been identified, has been removed from the field and faces possible charges.
Lt Gen John Sattler promised to pursue the facts of the case "thoroughly" before taking further measures.
The BBC's James Robbins says the incident could prove highly damaging and that the US military will need to answer key questions about whether the rules of engagement were broken during the incident.
It must explain, he says, whether wounded combatants were abandoned, or killed, illegally.
But the BBC's Baghdad correspondent Caroline Hawley says most Iraqis will not be surprised, after the scandal of US abuse of Iraqi prisoners at the Abu Ghraib facility in Baghdad.
US-led forces said they have now gained overall control in Falluja, trapping rebels in the south of the city.
The images of the alleged point-blank shooting of an Iraqi insurgent were taken by an NBC reporter embedded with the US troops in the Sunni city under assault.
They show a group of marines from the 3rd Battalion, 1st Regiment, armed with rifles, entering a building near a mosque last Saturday, 13 November.
The mosque had been used by insurgents to attack US forces, who had stormed it a day earlier, killing 10 militants and wounding five.
At least three severely wounded men are seen in a room inside the building - two are slumped against one of the walls, partially covered with a blanket.
The NBC's Kevin Sites says the wounded men had been left in the mosque after being treated by a group of marines following Friday's fighting.
Mr Sites says soldiers from a different unit went and apparently shot the men again on Saturday without knowing whether they were armed.
"Then one of the marines points his rifle at the head of one of the injured, an old man, saying, 'He's faking he's dead'," Mr Sites' description continues.
"The sound of a shot is then heard. And in the background, another soldier says, 'Well, he's dead now'."
The day before the incident, the marine who allegedly did the point-blank shooting had reportedly been shot in the cheek and returned to duty, while another marine in the same unit had been killed by explosives planted in the body of a dead insurgent.
Gen Sattler, commander of the 1st Marine Expeditionary Force, said the purpose of the investigation was to determine "whether the marine acted in self defence, violated military law or failed to comply with the law of armed conflict".
"We follow the law of armed conflict and we hold ourselves to a high standard of accountability," Gen Sattler said.
US marines have been battling with sporadic but fierce pockets of resistance inside the city. They say they have already killed 1,200 insurgents in the week-long assault. The offensive is now concentrated mainly in southern districts, where small groups of rebels are "fighting to the death", US commanders say. Aid agencies are warning of a humanitarian disaster in Falluja, which has been without water or electricity for a week.
FALLUJA, Iraq (Reuters) - U.S. Marines rallied round a comrade under investigation for killing a wounded Iraqi during the offensive in Falluja, saying he was probably under combat stress in unpredictable, hair-trigger circumstances.
Marines interviewed on Tuesday said they didn't see the shooting as a scandal, rather the act of a comrade who faced intense pressure during the effort to quell the insurgency in the city.
"I can see why he would do it. He was probably running around being shot at for days on end in Falluja. There should be an investigation but they should look into the circumstances," said Lance Corporal Christopher Hanson.
"I would have shot the insurgent too. Two shots to the head," said Sergeant Nicholas Graham, 24, of Pittsburgh, Pennsylvania. "You can't trust these people. He should not be investigated. He did nothing wrong."
The military command launched an investigation after video footage showed a U.S. Marine shooting a wounded and unarmed man in a mosque in the city on Saturday. The man was one of five wounded and left in the mosque after Marines fought their way through the area.
A pool report by NBC correspondent Kevin Sites said the mosque had been used by insurgents to attack U.S. forces, who stormed it, killing 10 militants and wounding the five. Sites said the wounded had been left for others to pick up.
A second group of Marines entered the mosque on Saturday after reports it had been reoccupied. Footage from the embedded television crew showed the five still in the mosque, although several appeared to be close to death, Sites said.
He said a Marine noticed one prisoner was still breathing.
A Marine can be heard saying on the pool footage provided to Reuters Television: "He's f***ing faking he's dead."
"The Marine then raises his rifle and fires into the man's head," Sites said.
NBC said the Marine, who had reportedly been shot in the face himself the previous day, said immediately after the shooting: "Well, he's dead now."
THOROUGH PROBE PROMISED
The Marine commander in Falluja, Lieutenant General John Sattler, said his men followed the law of conflict and held themselves to a high standard of accountability.
"The facts of this case will be thoroughly pursued to make an informed decision and to protect the rights of all persons involved," he said.
Marines have repeatedly described the rebels they fought against in Falluja as ruthless fighters who didn't play by the rules. They say the investigation is politically motivated.
"It's all political. This Marine has been under attack for days. It has nothing to do with what he did," said Corporal Keith Hoy, 23.
Rights group Amnesty International said on Monday both sides in the Falluja fighting had broken the rules of war governing the protection of civilians and wounded combatants.
Gunnery Sergeant Christopher Garza, 30, favored an investigation but like other Marines said the Pentagon (news - web sites) should weigh its decision carefully.
"He should have captured him. Maybe the insurgent had some valuable information. There may have been mitigating circumstances. Maybe his two buddies died in Falluja," he said.
Sites said: "I have witnessed the Marines behaving as a disciplined and professional force throughout this offensive. In this particular case, it certainly was a confusing situation to say the least."
The U.S. military has been embarrassed by scandals in Iraq (news - web sites), most prominently the Abu Ghraib affair in which at least eight U.S. soldiers have been tried or face courts-martial over the abuse of prisoners at the jail outside Baghdad.
There have also been several cases in which soldiers have been charged with wrongfully killing Iraqis during operations.
This whole war seems to be a computer game for many of them. They can be sure they pull on hatress from the Iraqis even more by that. The army acts like a dinosaur in a museum for a would-be democracy ...
By wiping out city by city the people will live on in war and poverty now. I great improvement to the situation just before the war, pfffff
I'm sure they find stupid excuses like shooting on a booby-trap with this man ('self-defense'). OMG the troops are a real model for the liberation fighters (to call them sometimes not terrorists).
forgive, you will never reach the level of my loyalty for the best that has ever played: