BAGHDAD, Iraq – The Shiite-dominated Interior Ministry announced an investigation Thursday into claims of death squads in its ranks as police found a dozen more bodies, bringing the number of apparent victims of sectarian reprisal killings here to at least 30 this week.

The probe was announced after U.S. military officials indicated there was evidence to support the allegation of death squads. The 12 men found on Thursday had been bound and shot in the head execution-style.

At least 25 other people were killed in violence across Iraq, including three tribal sheiks slain in a drive-by shooting north of the capital. Three supporters of anti-U.S. cleric Muqtada al-Sadr died in a Baghdad mortar barrage.

In the death squad case, Iraqi security officials said the Interior Ministry probe would focus on a single incident involving 22 Iraqi policemen who U.S. authorities said were detained last month before they were able to kill a Sunni Arab man north of Baghdad.

“They were dressed as Iraqi highway patrolmen but only four of those individuals were planning to conduct a kidnapping and subsequent murder of a Sunni individual,” Maj. Gen. Rick Lynch told reporters Thursday.

Maj. Gen. Joseph Peterson, who commands the civilian police training teams in Iraq, gave a slightly different version to the Chicago Tribune for a story Thursday about the death squad.

Peterson said the 22 men were wearing police commando uniforms but turned out to be employed by the Interior Ministry as highway patrol officers. He said four were believed to be ringleaders, while the 18 others were likely following orders. All were picked up at a checkpoint.

“We continue to believe that there’s more of these out there,” he said.

But Lynch said this could have been an isolated incident.

“We have no indication they have done this in the past,” he said. “All we know this is an isolated incident.”