The official, who is close to the investigation, told The Associated Press that 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.
U.S. 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 6 miles north of Mahmoudiya, he said.
There were some discrepancies over how many soldiers were being investigated. The U.S. official said it was at least four. Two other U.S. officials said Friday that five were under investigation but one already had been discharged for unspecified charges unrelated to the killings and was believed to be in the United States.
In Baghdad, the U.S. military issued a terse statement Friday, saying only that Maj. Gen. James D. Thurman, commander of the 4th Infantry Division, ordered a criminal investigation into the alleged slaying of a family of four in Mahmoudiya, 20 miles south of Baghdad.
The four still in the Army have had their weapons taken away and are confined to a U.S. base near Mahmoudiya, officials said. If convicted of premeditated murder, the soldiers could receive a death sentence under U.S. military law.
The suspects in the killing were from the same platoon as two soldiers kidnapped and killed south of Baghdad this month, another official close to the investigation said Friday. Their mutilated bodies were found June 19, three days after they were abducted by insurgents near Youssifiyah southwest of Baghdad.
The military has said one and possibly both of the slain soldiers were tortured and beheaded. The official said the mutilation of the slain soldiers stirred feelings of guilt and led at least one member of the platoon to reveal the rape-slaying on June 22.
One soldier was arrested after admitting his role in the alleged attack on the family, he said on condition of anonymity because the case is under way. The official said the rape and killings appear to have been a "crime of opportunity," noting that the soldiers had not been attacked by insurgents but had noticed the woman on previous patrols.
One of the family members they allegedly killed was a child, said a senior Army official who also requested anonymity because the investigation is ongoing. The senior official said the alleged incident was first revealed by a soldier during a routine counseling-type session. The official said that soldier did not witness the incident but heard about it.
A second soldier, who also was not involved, said he overhead soldiers conspiring to commit the crimes and then later saw bloodstains on their clothes, the official said.
The allegations of rape could generate a particularly strong backlash in Iraq, a conservative, strongly religious society in which many women will not even shake hands with men who are not close relatives.
The case is among the most serious against U.S. soldiers allegedly involved in the deaths of Iraqi civilians. At least 14 U.S. troops have been convicted.
In other developments:
The accused soldiers were from the same platoon as privates. The military has said one and possibly both of the slain soldiers were tortured and beheaded.
Last week, seven Marines and one Navy medic were charged with premeditated murder in the shooting death of an Iraqi man near Fallujah west of Baghdad.
U.S. officials are alsoNov. 19 in the western town of Haditha in a revenge attack after a fellow Marine died in a roadside bombing.
Other cases involve the deaths of three male detainees in Salahuddin province in May, the shooting death of unarmed Iraqi man near Ramadi in February, and the death of an Iraqi soldier after an interrogation in 2003 at a detention camp in Qaim.
The official said the mutilation of the slain soldiers stirred feelings of guilt and led at least one of them to reveal the rape-slaying on June 22.
According to a senior Army official, the alleged incident was first revealed by a soldier during a routine counseling session. The official, who requested anonymity because the investigation is ongoing, said that soldier did not witness the incident but heard about it.
A second soldier, who also was not involved, said he overhead soldiers conspiring to commit the crimes, and then later saw bloodstains on their clothes, the official said.
He also said the four people killed included three adults and a child, and one of the adults was the woman who allegedly was raped.
One of the accused soldiers already has been discharged and is believed to be in the United States, several U.S. officials said on condition of anonymity because the investigation is ongoing. The others have had their weapons taken away and are confined to Forward Operating Base Mahmoudiyah.