Confirming another major investigation into alleged killing and abuse of Iraqi civilians by at least three US soldiers, one officer said the incident in March just south of Baghdad had initially been blamed on fighters active in the area.
The inquiry is the latest in a series launched in recent weeks since the revelation of investigations into the killings of 24 people in Haditha in November.
Officers said a criminal inquiry was launched on June 24 into whether soldiers killed four people in their home at Mahmudiya on March 12.
Police in the district said they could not recall a case meeting the description given by the US military.
Major-General James Thurman, the commander of the Baghdad area, ordered the army's Criminal Investigation Command to mount a full investigation within a day of two soldiers coming forward, the military said in its brief initial statement.
Major Todd Breasseale, a US military spokesman, said in Baghdad: "We're not going to leave any stone unturned."
In Washington, an army official said the suspicion was that two soldiers from the 502nd Infantry Regiment, later seen with blood on their uniforms, had raped a young woman and then one of them killed her and three of her family, among them a child.
"We're not going to leave any stone unturned"
Major Todd Breasseale,
US military spokesman
At least one other soldier was being investigated.
The suspected killer had been discharged from the army and was in the United States.
Officials have said in similar cases that discharged soldiers can be recalled to face a court martial.
Last week, 12 American soldiers were charged with murder in two separate cases of killing civilians and another was charged with voluntary manslaughter in a third.
Charges are expected to be brought soon in the Haditha case.