A former US Army soldier, sentenced to life imprisonment for the 2006 rape and murder of a 14-year-old Iraqi girl and the killing of her parents and sister, has been found hanged in his cell.

The Los Angeles Times report, quoting prison officials, said the death of Steven Dale Green was being investigated as suicide. Green had been found hanging in his Arizona cell last week, according to the Times report, which was published on Tuesday.

Green, 28, was convicted in 2009 of the rape and murder of 14-year-old Abeer Qassim Hamza al-Janabi and the deaths of her father, mother and six-year-old sister in Mahmudiya, 32km south of Baghdad.

He was sentenced to life in prison, without the possibility of parole, after a federal jury in Kentucky could not decide whether he should be executed.

During the trial, prosecutors portrayed him as the ringleader of a gang of five soldiers that plotted to invade the home of the family of four to rape the girl, and later bragged about the crime.

Green, who was 19 when he committed the crime, was described as the triggerman in the group of soldiers, who donned black "ninja" outfits and raped the girl before killing her and her family.

Three of the four other soldiers pleaded guilty in the attack and the fourth was convicted, all in military courts. They received sentences ranging from five to 100 years. Green was tried as a civilian because he was arrested after he was discharged from the army. He was described by prosecutors as predisposed to killing Iraqis.

While his defence lawyers acknowledged that he took part in the killings, they argued he was suffering combat stress after the death of close colleagues and should be spared the death penalty.