BAGHDAD (AP) – The U.S. military said Friday it was investigating the deaths of three civilians who were shot by American troops near a checkpoint set up by Iraqis who have joined forces against extremists.

The civilians were killed early Thursday in Abu Lukah, a village just north of Musayyib, about 60 kilometers (40 miles) south of Baghdad, the military said. The brief announcement did not identify the civilians by sect or provide other details. But a local police spokesman said those killed were Shiite members of the North of Hillah Awakening Council, a group of Iraqis who have turned against extremists in the area.

Five council members were guarding a deserted road into their village at about 2 a.m. when U.S. troops fired on them from a watchtower at a nearby military base, the spokesman said, speaking condition of anonymity because he was not authorized to release the information.

Three guards were killed, another was wounded and the fifth man fled on foot, the official said.

The U.S. military is relying heavily on the growing number of Sunni tribes that have turned against al-Qaeda, saying their support is key to a secure Iraq. It also has claimed success in extending the movement to Shiites opposed to growing lawlessness among militia factions.

The Americans point to successes in the Anbar province, which is now largely peaceful after Sunnis joined the Iraqi military and police force as a way to both protect themselves from extremists and to empower them in the face of the Shiite-led central government in Baghdad.