Eight members of an armed civilian group paid by the U.S. military were killed when trying to turn over a roadside bomb to U.S. forces in northern Iraq, officials said Wednesday.

Three other members of the “Sons of Iraq” group were wounded in the incident, U.S. officials said. The explosion occurred in southern Ninevah province on Tuesday, when “members were transporting the [roadside bomb] to a nearby joint combat outpost for destruction,” a U.S. military statement read.

Such groups have been one of the linchpins for the security gains in Iraq that were seen for much of last year. The groups — which are mainly Sunni and include some 90,000 people throughout the country — are paid monthly salaries by the U.S. military but operate on their own control.

They are mainly used to guard neighborhood checkpoints, though they also search for explosives and deliver them to U.S. forces for disposal — sometimes for cash.

The incident on Tuesday is under investigation, the U.S. military said.

“These individuals helped save the lives of their fellow citizens,” Maj. Dan Meyers, a U.S. military spokesman in northern Iraq, said in Wednesday’s news release. “We send our condolences to the families of the brave Sons of Iraq members who continue to stand up against those who harm their fellow Iraqis.”

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