Some of the five soldiers also allegedly burned the body of the woman they are accused of assaulting in the March incident, a U.S. military official told The Associated Press on condition of anonymity because of the sensitivity of the case.
The U.S. command issued a statement saying only that Maj. Gen. James D. Thurman, commander of coalition troops in Baghdad, had ordered a criminal investigation into the alleged killing of a family of four in Mahmoudiyah, south of Baghdad.
At least 14 American troops have been convicted in other cases.
The United States also is investigating allegations that two dozen unarmed Iraqi civilians werein the western town of Haditha on Nov. 19 in a revenge attack after one of their own died in a roadside bombing.
"The entire investigation will encompass everything that could have happened that evening. We're not releasing any specifics of an ongoing investigation," military spokesman Maj. Todd Breasseale said of the Mahmoudiyah allegations.
"There is no indication what led soldiers to this home. The investigation just cracked open. We're just beginning to dig into the details."
Pentagon spokesman Bryan Whitman said he had no additional details on the incident but added that the military routinely investigates all allegations of misconduct.
However, a U.S. official close to the investigation said at least one of the soldiers, all assigned to the 502nd Infantry Regiment, has admitted his role and been arrested. Two soldiers from the same regiment were slain this month when they were kidnapped at a checkpoint near Youssifiyah.
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.
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.
Senior officers were aware of the family's death but believed it was due to sectarian violence, common in the religiously mixed town, a U.S. official said.
The killings appeared to have been a "crime of opportunity," the official said. The soldiers had not been attacked by insurgents but had noticed the woman on previous patrols.