( CNN )- Six Iraqis were killed Saturday when a U.S. helicopter fired on a small gathering at what may have been a pro-U.S. group's checkpoint, officials said.
A Samarra police official said the helicopter "mistakenly" hit a Sons of Iraq checkpoint, killing the six. Two other Iraqis were wounded in the attack in Ashaki, south of Samarra, 55 miles north of Baghdad.
The U.S. military said that after five people were "spotted conducting suspicious terrorist activity" near a recent roadside bombing site, an AH-64 Apache fired on them. It was unclear how the other dead and wounded were involved.
The Sons of Iraq are one of many groups generically referred to as Awakening Councils -- largely Sunni local security groups that have been recruited by the U.S. military.
A U.S. patrol stopped and talked with the Sunni militiamen about two hours before the attack, a local Awakening Council leader told The Associated Press.
"They asked us general questions like: 'Have you gotten your IDs?' and 'Do you need anything?' and then they left," Sabbar al-Bazi told AP. "Two hours later, after I had gone home, I heard two explosions, probably caused by two missiles, and machine-gun fire from a helicopter."
The U.S. and Iraqi militaries are conducting a joint investigation, a U.S. spokesman said.
Meanwhile, a series of roadside bombings Saturday killed three U.S. soldiers and four Iraqi civilians and wounded several other people.
The soldiers were killed by a bomb that struck their vehicle while they were on patrol in northwestern Baghdad, the U.S. military said.
Two Iraqi civilians were also killed in that attack, the military said.
A U.S. soldier also was killed Friday, the military said, by indirect fire south of Baghdad. Four other soldiers were wounded.
Although the military gave no further details, "indirect fire" is the term the military uses for rockets or mortars.
The military deaths bring to 3,996 the number of U.S. service members who have died in the Iraq war. Eight civilians working for the Defense Department are included in that number.
On Saturday, a roadside bomb exploded near an Iraqi police patrol in Kirkuk, killing a civilian and wounding nine people, including five police officers, a Kirkuk police official said.
Kirkuk is about 150 miles north of Baghdad.
A roadside bomb killed a civilian and wounded eight in a Shiite neighborhood in eastern Baghdad. Another roadside bombing killed an Awakening Council member and wounded five other people in western Baghdad's affluent Mansour district, according to an Iraqi Interior Ministry official.
Another roadside bomb near a U.S. military convoy in southern Baghdad's Dora district wounded three bystanders, the official said.