A group of American soldiers in an insurgent-riddled town allegedly noticed a young Iraqi woman when on patrol and later returned to rape her, according to U.S. officials Friday. In an apparent cover-up attempt, she and three members of her family then were killed and her body was set on fire.

Five U.S. troops are being investigated, a U.S. military official told The Associated Press.

It is the fifth pending case involving alleged slayings of Iraqi civilians by U.S. troops.

The suspects in the killing, which took place in March, were from the same platoon as two soldiers kidnapped and killed south of Baghdad this month, said the official, who is close to the investigation and spoke on condition of anonymity because of the sensitivity of the case.

One soldier was arrested after admitting his role in the alleged attack on the family, the U.S. official said. 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. Some of the suspects allegedly burned the woman's body to cover up the attack, the U.S. official said.

In Baghdad, the U.S. military issued a sparse statement, 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.

However, the U.S. official said the soldiers were assigned to the 502nd Infantry Regiment. The official told the AP that the suspects were from the same platoon as two slain soldiers whose 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.

According to the senior Army official, 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.

Before the soldier disclosed the alleged assault, senior officers had been aware of the family's death but believed it was a result of sectarian violence, the official said.

One of the five suspects has already been discharged for unspecified charges unrelated to the killings and is believed to be in the United States, two 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 a U.S. base near Mahmoudiya.

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.

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 also investigating allegations that U.S. Marines killed two dozen unarmed Iraqi civilians Nov. 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 allegations have aroused public anger against the U.S. military presence at a time when the new Iraqi government and U.S. authorities are trying to reach out to disaffected Sunni Arabs to quell the insurgency and calm sectarian tensions.

On Saturday, Prime Minister Nouri al-Maliki leaves for a whirlwind trip to Saudi Arabia, Kuwait and the United Arab Emirates to seek support for his national reconciliation initiative, which includes an amnesty for the mostly Sunni insurgents.

Al-Maliki is also expected to brief the Sunni leadership of those three countries on his efforts to deal with the divisions between Shiites and Sunnis. Iraq's neighbors in the Persian Gulf fear sectarian tensions will spill over into their countries, which are dominated by Sunnis but have large Shiite minorities.

On Friday, radical Shiite cleric Muqtada al-Sadr rejected al-Maliki's initiative because it does not include a timetable for the withdrawal of U.S.-led foreign troops.

"We demand the occupation forces to leave the country, or at least a timetable should be set for their withdrawal," al-Sadr said during a sermon.

Despite al-Maliki's efforts, there has been no letup in Iraq's violence. The U.S. military reported four more American service members have died, including a Marine killed Friday in fighting west of Baghdad. Three Army soldiers died in combat the day before, the military said.

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AP correspondent Ryan Lenz is embedded with the 101st Airborne Division in Beiji, Iraq. He was previously embedded with the 502nd Infantry Regiment in Mahmoudiya. AP correspondent Lolita C. Baldor in Washington contributed to this report.