Investigators believe the US soldiers suspected of raping an Iraqi woman, then killing her and members of her family plotted the attack for nearly a week, a US military official said on Saturday.
The official, who is close to the investigation, said flammable liquid was used to burn the woman's body in a cover-up attempt, although it was not clear if it was gasoline or lighter fluid.
The official, speaking on condition of anonymity because the investigation is ongoing, said it appeared the attack was "totally premeditated" and that the soldiers "studied them for about a week."
According to the official, the Sunni Arab family had just moved into a new home in the insurgent-riddled area around Mahmoudiya, south of Baghdad.
The Americans entered the home, separated three males from the woman, then raped her and set fire to her body, the official said. The three males were also slain.
US officials said they knew of the deaths but thought the victims died due to sectarian violence.
But Mahmoudiya police Capt Ihsan Abdul-Rahman said Iraqi officials received a report March 13 alleging that American soldiers had killed the family.
The incident occurred in the Khasir Abyad area, about 9 kilometres north of Mahmoudiya, he said.
There were some discrepancies over how many soldiers were being investigated.