An army official said the criminal investigation was ordered after two other soldiers said they had heard about -but not witnessed - the alleged killing in March in the area of Mahmudiyah, south of Baghdad.
"Allegations known so far are that two soldiers allegedly raped an Iraqi woman and then allegedly one of the soldiers may have killed four Iraqi civilians located within that residence - the woman, two other adults and a child," the official said.
Officials could not confirm reports that the soldiers also were alleged to have set fire to the house.
The soldiers belonged to the same 1st Battalion of the 502nd Infantry Regiment as Kristian Menchaca and Thomas Tucker, whose "severely traumatized" bodies were found June 19 near a canal in Yusufiyah.
The official said the rape and murder allegations came to light last month during a "combat stress debriefing" of the unit after Menchaca and Tucker's deaths.
An enlisted soldier disclosed he had heard that as many as three soldiers from the unit may have taken part in a rape and murder in March.
The US military has come under the spotlight over a number of incidents of abuse of Iraqis since the US-led invasion in March 2003, most notoriously the mistreatment of detainees at Abu Ghraib, which prompted a string of convictions.
The military is also investigating allegations that US marines killed 24 civilians, including 10 women and children, in the Euphrates valley town of Haditha last year after a bomb killed a comrade.
Al-Qaeda reward
US Secretary of State Condoleezza Rice has authorised a reward of up to five million US dollars for information leading to wanted Al-Qaeda leader Abu Ayyub al-Masri, in Iraq.
In a statement, Deputy State Department spokesman Adam Ereli said that the reward was offered in the hope it would encourage persons to come forward with information on al-Masri.
The State Department said Abu Ayyub al-Masri is an Egyptian national, a senior Al-Qaeda leader in Iraq and a direct associate of Abu Musab al-Zarqawi, who was recently killed by US forces.
The reward was offered as a CIA official said an audiotaped message attributed to Al-Qaeda leader Osama bin Laden and which threatened new attacks was authentic.
Mr Ereli later told a press briefing: "This tape demonstrates that we still have our work cut out for us".
“(Al-Qaeda's) capabilities have been lessened, but they still remain a threat. The fight against al-Qaeda and others who practice terrorism is going to be a long one."
Russia offers reward
Russia has also offered a 10-million-dollar reward for information leading to the elimination of those who killed four Russian embassy employees in Baghdad.
Earlier this week Russia’s President Vladimir Putin reportedly told Russian secret services to "take all necessary steps to find and destroy the criminals who committed this evil deed".
The embassy employees were abducted when gunmen attacked their vehicle in the west Baghdad neighbourhood of Mansur on June 3. A fifth Russian citizen was killed during the kidnapping.
Iraq PM goes abroad
Iraq Prime Minister Nuri al-Maliki is embarking on his first trip abroad to garner support for his much-touted peace plan that has been rejected by the Sunni leaders and rebels.
Mr Maliki will visit Saudi Arabia, the United Arab Emirates and Kuwait.