An Iraqi army medic has told a military hearing in Baghdad how he found the bodies of four Iraqis allegedly murdered by US troops.
The legal hearing could go on for several days
The preliminary hearing will establish if there is sufficient evidence to court martial four American soldiers.
The men are accused of raping and killing an Iraqi girl and murdering three of her family members in Mahmudiya, south of Baghdad.
It is one of a series of atrocities being blamed on US forces in Iraq.
Sergeant Paul Cortez, Specialist James Barker, Private Jesse Spielman and Private Bryan Howard are charged with conspiring to rape the girl in the attack on 12 March after shooting dead her parents and five-year-old sister.
Iraqi witnesses are giving evidence in a closed session because of the risk that they could be targeted by insurgents for appearing to help the American military in its prosecution.
The BBC's Jane Peel, on the military base in Baghdad where the hearing is taking place, says the Iraqi medic described arriving on the scene of the deaths.
He told the hearing that in the living room of the family's home was the body of a 14-year-old girl. He said she had been burnt from the waist up.
Elsewhere in the house, he discovered three other bodies: a woman who appeared to have been shot in the chest, a man shot in the head and a little girl with a bullet wound in her face.
The medic told prosecutors he was ill for weeks after witnessing the crime scene.
The four accused are alleged to have helped a former private - Steven Green, who has since left the army - plan, carry out and cover up the attack. Mr Green has pleaded not guilty in a federal court and will be tried separately in the United States.
The lawyer for Spc Barker said the stressful environment in the Mahmudiya area contributed to the soldiers' behaviour, according to the Associated Press news agency.
Series of scandals
The soldiers are appearing before an Article 32 hearing - the military equivalent of Grand Jury proceedings in the American civilian courts - in proceedings expected to last several days.
If subsequently prosecuted at a court martial and found guilty, those accused of murder could be given the death penalty, our correspondent adds.
The case is the latest in a series of scandals that has tarnished the reputation of US troops battling Iraq's violent insurgency.
Two US military inquiries have been looking into an alleged massacre of 24 Iraqi civilians by US marines last November in the town of Haditha.
Days ago, a military prosecutor branded four US soldiers accused of killing Iraqi detainees in Samarra "war criminals" as he pressed for them to face a court martial at another Article 32 hearing.
Seven US marines and a navy sailor have also been charged over the death of a disabled Iraqi man on 26 April in Hamdaniya.