Police arrested at least eight people on Thursday in connection with the murder of 22 Shiite Muslim pilgrims en route to Syria from a holy city in south Iraq, officials said.
The victims had been passengers on a bus on Monday evening from the shrine city of Karbala passing through Anbar province, long a stronghold of Sunni insurgents and Al-Qaeda's front group in Iraq, when their vehicle was stopped by gunmen and they were killed.
"Elite police from Karbala arrested eight suspects over the crime in Nukhaib," said Nusaif Jassim al-Khitabi, deputy chief of Karbala provincial council, referring to the area where the attack took place.
"They are still being held. They were all arrested in Anbar."
An official in Anbar's provincial council, meanwhile, said 20 people, including two clerics, were detained and were all still being held.
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In Monday's attack, gunmen dressed in military and police uniforms set up a fake checkpoint and made the bus passengers disembark before killing all the men and fleeing the scene, officials said.
The bus was one of the daily services departing from Karbala carrying Shiite pilgrims bound for Syria. On its way to Iraq's western neighbour, it must pass through the desert region of Anbar.
Since the US-led invasion of 2003, mainly Sunni Anbar has been a stronghold of Al-Qaeda, whose members have killed numerous Iraqis and foreigners travelling the roads to Jordan and Syria.
While tribal militias have cracked down on the insurgents since 2007, they have not completely eliminated them.
Also on Thursday, Ahmed Abu Risha, whose brother founded the first Sunni tribal militia that joined forces with the US military against Al-Qaeda, called on Iraqi Prime Minister Nuri al-Maliki to hand over responsibility for security in Anbar to the militiamen, known as the Sahwa.
He told Al-Sharqiya television that Iraqi security forces could not handle security in the province, and the Sahwa should take over instead.