The US Marine accused of leading the killing of as many as 24 unarmed Iraqi civilians in the worst alleged atrocity by American forces during the war in Iraq faces a military hearing today that will decide whether he is to be court-martialled.

Sergeant Frank Wuterich, 27, told military investigators that he ordered his squad of soldiers to "shoot first and ask questions later" after a powerful roadside bomb killed one of their comrades in the volatile town of Haditha, westerm Iraq, on November 19, 2005.

In the gunfire that followed, Iraqi witnesses said 24 civilians, including women and children inside a house, were shot dead, many at close range. A US military statement reported that 15 people had died in a roadside bombing.

When an Iraqi human rights group made public allegations of war crimes, Sergeant Wuterich and three other Marines were charged with murder and four commanding officers were accused of helping them cover up their alleged crimes.

The alleged massacre at Haditha has joined the prisoner abuse scandal at Abu Ghraib as one of the most damaging incidents of the Iraq war for US forces.

At today's hearing at Camp Pendleton, California — known as an Article 32 hearing — an investigating officer, Lieutenant Colonel Paul Ware will hear evidence to decide whether Sergeant Wuterich broke the US military's rules of engagement during the episode. He is accused of murdering 18 people and faces life imprisonment if convicted by a court martial.

Colonel Ware has already recommended that charges be dropped against another Marine facing a murder charge because of the "shadowy" nature of the fighting in Haditha. Charges have already been dropped against a third, while the fourth, Sergeant Sanick de la Cruz, has offered to testify against Sergeant Wuterich in return for having the charges against him reduced.

Central to the hearing will be Sergeant de la Cruz's allegation that Sergeant Wuterich shot dead five Iraqis who approached the Marines in Haditha with their hands raised, trying to surrender.

“They were just standing, looking around, had hands up,” De la Cruz told a hearing in May. “Then I saw one of them drop in the middle. I didn’t know what was going on, sir. Looked to my left, saw Staff Sergeant Wuterich shooting.”

Sergeant Wuterich is then accused of ordering the "clearing" of a house near where the roadside bomb exploded, during which several members of the same family were shot. He faces an additional murder charge in connection to the deaths of four men shot later in another building.

His legal team is expected to argue that he stayed within the rules of engagement and use forensic evidence to challenge Sergeant de la Cruz's version of events. "The argument is quite simple, that the Marines that day were reacting to a hostile and dynamic environment and that their conduct, although it led to tragic results, was completely lawful," Mark Zaid, an attorney on Sergeant Wuterich's legal team told Reuters.

“The civilians who died were collateral damage as part of a combat operation where insurgents would intentionally place civilians into harm’s way,” said Mr Zaid.

In testimony before a hearing in February 2006, Sergeant Wuterich told investigators that he ordered his men to treat the house close to the explosion as "hostile", meaning his men did not have to determine whether the occupants were insurgents before killing them.

“I told them to shoot first, ask questions later,” he said.

The hearing is expected to last two days.

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