A female suicide bomber on Monday killed a sheik who led a U.S.-aligned armed civilian group in Iraq’s Diyala province, the latest in a series of such attacks, officials said.

The attack occurred around 8 a.m. in the home of Sheik Thaeir Ghadhban al-Karkhi in Kanaan, about 12 miles east of Baqouba, according to Iraqi police and other officials. The sheik led a mainly Sunni citizens groups enlisted by coalition troops to help enforce security.

Rear Adm. Gregory Smith told reporters in Baghdad on Sunday that more than 90,000 Iraqis have joined such groups, which the U.S. military has credited for helping tamp down violence in the country, CNN reported.

There was no immediate claim of responsibility, but al-Qaida in Iraq has targeted Iraqis who have taken up arms against insurgents and joined groups such as the one led by al-Karkhi.

Duraid Mahmoud, the sheik’s brother, said the bomber had visited the sheik’s house on Sunday, claiming her husband had been kidnapped and she needed help, The Associated Press reported. Mahmoud said his brother told her to return Monday.

“She came back this morning and nobody checked her,” Mahmoud said.

The woman detonated a bomb, killing al-Karkhi, his 5-year-old niece, a 24-year-old cousin and a security guard.

About an hour later, a male suicide bomber killed two civilians and wounded 20 people, including two police officers, in Muqdadiyah, north of Baghdad, according to CNN.

At least 100 members of citizens groups that help U.S. maintain peace were killed by assassinations and bomb attacks in January, mostly around Baghdad and Baqouba, Interior Minister Jawad al-Bolani was quoted as saying by The New York Times.

Iraqi officials blame some of the attacks on Shiite paramilitary forces and/or splinters of the groups, and even on Iran, according to the Times report.

In Anbar province in western Iraq, where the groups first formed, a teenage suicide bomber exploded last month at a gathering of tribal leaders, killing Sheik Hadi Hussein al-Issawi and three other tribesmen.

Last September in Anbar, a popular Sunni leader named Sheik Abdul Sattar Abu Risha and two others were killed in a two-bomb explosion on his family compound.

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