A mosque in the Iraqi city of Fallujah blew up yesterday, killing a virulently anti-American Sunni cleric who had been preaching jihad against coalition forces. Surprise! It seems the jihadis were making bombs in the religious study room. Mosque blast kills outspoken cleric. (Hat tip: Joel G.)

THE CAUSE of the blast at the Hassan mosque Monday night has not been determined. U.S. officials said an investigation was underway.

At least one Fallujah resident and some U.S. soldiers suggested that the victims of the blast were manufacturing explosives in a three-room cinder-block building next to the mosque. Hospital officials said that the extensive burns suffered by the victims and lack of shrapnel wounds suggested that explosive or incendiary material had detonated accidentally inside the building.

That explanation, if proven, would be the clearest indicator yet of involvement by Islamic activists in the simmering guerrilla war in Iraq, adding another dimension to a conflict that U.S. officials say has been driven almost entirely by remnants of ousted president Saddam Hussein’s avowedly secular government.

Relatives and angry residents, however — some chanting “America is the enemy of God” — blamed the explosion on U.S. forces, and some said they saw the flash of a missile or heard the drone of aircraft immediately before the blast.

U.S. officials denied they were responsible. “We did not bomb it,” said Capt. John Ives of the 3rd Infantry Division in Fallujah, 35 miles west of Baghdad. “It was not dropped by an airplane, nor was it dropped by a helicopter, nor was it fired by a tank. The blast did not come from an external source.”

The blast devastated the mosque compound, which is bordered by palm and fig trees. The explosion blew off a 30-foot chunk of the outside wall, carved a 10-foot-high hole into the mosque’s facade and shattered windows of nearby houses. The front of a carpentry shop was torn off. Nothing remained of the part of the building where the blast occurred-a room for the mosque custodian, another room for religious study and a closet for the generator. Piles of rubble and tangled steel rods littered the courtyard.

A group of Religion of Peace™ members took the dead cleric’s body from the hospital—and threatened to come back:

Outside Baghdad’s Jarrah Private Hospital, a group of 30 of Khalil’s followers, many of them in turbans and white dishdashas, or long tunics, waited in the lobby for word on the cleric’s fate. Anis Akram, a physician at the hospital who informed them of his death, said at least one spoke in Syrian-accented Arabic. They took Khalil’s body for burial in a convoy of four gray minibuses.

Before they left, Akram recalled, “they threatened to attack the hospital.”

UPDATE: Latest word is that the blast happened during a bomb-making class.

“The explosion was apparently related to a bomb manufacturing class that was being taught inside the mosque,” the U.S. Central Command said in a statement. Falluja residents had accused U.S. forces of attacking the mosque late Monday.

Residents said the blast killed nine people, including the mosque’s imam, or prayer leader.