WASHINGTON -- U.S. Army officials are investigating allegations that U.S. soldiers killed several detainees after they were captured on a battlefield southwest of Baghdad last year, officials said Tuesday.

Military officials said the incident under review took place about six months ago in the Rashid district and involved soldiers from the 2nd Brigade, 1st Infantry Division. The brigade, which is based in Germany, was deployed to Iraq from September 2006 to November 2007.

The detainees allegedly were killed at the point of capture and were never brought back to a U.S. or Iraqi base for questioning, said Paul Boyce, a U.S. Army spokesman. The military released no details regarding how many soldiers were involved, how many detainees and who made the allegations. Frequently, such allegations surface months later, when soldiers come forward or talk among themselves.

Soldiers from at least one company of the 2nd Brigade were questioned by investigators in Germany last week. The investigation is being led by the Army’s Criminal Investigations Command. In Iraq on Tuesday, the northern city of Mosul was hit with another day of high-profile violence when a suicide bomber detonated his car near a U.S. convoy. One Iraqi was killed and 15 others were injured, Iraqi police and the U.S. military said.

The bombing came one day after five U.S. soldiers were killed in a roadside attack and less than a week after as many as 60 people were killed by an explosion at a booby-trapped apartment building.

To the south, in Muqdadiya, security officials said they found nine corpses and 10 severed heads in the countryside. The nine corpses, all male, were found blindfolded, handcuffed and shot in the head. The 10 heads also were found blindfolded, Diyala police officials said.

U.S. military officials have long struggled to gain control of Diyala, a stronghold of al-Qaida in Iraq. U.S. troops helped take back the provincial capital of Baqouba last year, only to drive insurgents into outlying communities such as Muqdadiya. Earlier this month, thousands of troops descended on the Diyala River Valley in an effort to strike at the militant group, which military officials say is foreign-led but whose foot soldiers primarily are Iraqis.

In Thar Thar, north of Fallujah in western Anbar province, Iraqi police reported clashes between a group of gunmen and members of the Saqlawiya Awakening Council. Police said 20 gunmen were killed in the battle; a member of the U.S.-backed security force of local citizens said 22 people were killed.

“We were trying to capture a group of terrorists who had a stronghold there and were robbing and killing citizens driving along that road,” said Alahi Ahmed, a member of the Awakening Council. “Our aim was to secure the pathway and cleanse it from these thieves.”

Barnes reported from Washington, and Yoshino from Baghdad. Special correspondents in Mosul, Ramadi and Baghdad contributed to this report.