Gunmen killed at least 13 people in a Sunni village west of Baghdad, dumping their bullet-riddled bodies in a cemetery, Iraqi security officials said Monday.

The motive for the attack was unclear, but could be a case of insurgents killing locals allied to the central government or an internal struggle among the region's fractious tribes. One unconfirmed report even suggested the gunmen wore military uniforms.

The dead included relatives of a leading figure in the local branch of the influential Iraqi Islamic Party, according to local police official Waleed al-Zubaei.

He added that all 13 had been shot in the head, possibly execution-style, as well as elsewhere in their bodies.

Violence dropped dramatically in the predominantly Sunni regions west of Baghdad after local tribes banded together and turned on al-Qaida in Iraq. Revenge attacks against government-allied politicians by insurgents and their families do still take place, however.

Maj. Gen. Qassim al-Moussawi, head of Baghdad's Operations Command, said in a statement that officials are looking into the 13 deaths and suspect it may have been a tribal dispute.

Mohammed Sadoun, an official who works at Abu-Ghraib hospital where the bodies were taken, told The Associated Press on Monday that at least seven bullet-riddled corpses were brought to the hospital in the morning.

A curfew has been imposed on the area and Iraqi military vehicles are patrolling the area, said al-Zubaei.

A witness who did not want to be identified for fear of retribution, said he went to the cemetery where the bodies were dumped and saw at least 12 bodies, ranging in age from 25 to 50 years old.