More info comes out. Leaning towards the Shia "cult" theory now, as opposed to Sunni insurgents as the Najaf governor originally said. Also the presence of families amongst the "insurgents." I wonder at the proportion of women and kids to gunmen.

Women and children who joined 600-700 of his "Soldiers of Heaven" on the outskirts of the Shi'ite holy city may be among the casualties, Shirwan al-Waeli told Reuters. All those people not killed were in detention, many of them wounded.

...

Some of the fighters wore headbands describing themselves as "Soldiers of Heaven," Iraqi officials said. It was not clear how many women and children were present: "It is very sad to bring families onto the battlefield," Waeli said.

When police first approached the camp and tried to call on the group to leave, their leader replied: "I am the Mahdi and I want you to join me," Waeli said, adding: "Today was supposed to be the day of his coming."

Other Iraqi officials said on Sunday that a man named Ahmed Hassani al-Yemeni, who had been working from an office in Najaf until it was closed down earlier this month, had assembled the group, claiming to be the messenger of the Mahdi.
We'll see how this plays out eventually. I sure hope American tanks weren't called in to do SCIRI's dirty work.