Baghdad, Iraq ? A video posted by an Iraqi insurgent group Sunday purported to show last week’s suicide attack at a U.S. base in Mosul, with a fireball rising from a white tent. The group claimed that the bomber slipped into the base through a hole in the fence during a guard change.

The footage showed a black-garbed gunman wearing an explosives belt around his body — apparently the suicide bomber, identified in the tape as Abu Omar al-Mosuli — bidding farewell to his comrades. The video gives no further details about the bomber beyond his name.

The Ansar al-Sunnah Army had earlier said it would release a video of Tuesday’s attack, which killed 22 people, including 18 U.S. service members and civilian contractors.

The bombing, the deadliest attack on a U.S. base in Iraq, has prompted a U.S. military investigation into how the bomber got onto the heavily guarded site and how security at bases can be improved. Three Iraqi National Guardsmen and a fourth “non-U.S. person” also were killed. The military has not said whether that fourth man was the bomber.

The U.S. military has said the attacker probably was wearing an Iraqi military uniform, and one general said the Iraqi security forces might have been infiltrated. The Iraqi chief of staff, Gen. Babaker B. Shawkat Zebari, said in an interview with The Associated Press that the bomber might have bought a uniform from the market but was not a member of the Iraqi security forces.

In the first section of the video — with a time signature of Dec. 20, a day before the attack — three gunmen wearing black masks and clothes and holding automatic rifles are shown sitting in front of a black banner with the group’s name on it. One of them, apparently al-Mosuli, sits on the left, wearing an explosives belt.

The gunman in the center reads a statement describing how the attack will be carried out. No mention is made of wearing a uniform. The authenticity of the video could not be independently verified.

“One of the lions from our martyrdom-seeking brothers will infiltrate the defenses of the enemy at the Morez base in Mosul. He will slip through a hole in the camp’s wire, exploiting the changing of the guard,” the gunman said. “We have been observing their schedule for a long time.”

“This lion will then proceed to his target, and he will take advantage of lunch time, when the dining hall is crowded with the crusaders and their (Iraqi) allies,” he said. “The operation will then be carried out.”

“Let Bush, Blair and Allawi know that we are coming and that we will chase them all away, God willing,” he said, referring to President Bush and prime ministers Tony Blair of Britain and Ayad Allawi of Iraq.

The two men then embrace the one wearing the explosives belt.

An image then shows a map of the base, as one of the gunmen points out locations using a military knife. One location is marked “the dining hall” in Arabic.

A later outdoor video image — shot on Tuesday, when the attack occurred — shows a fireball rising from the distance with the accompanying sound of the explosion. A final image, shot from a vehicle driving past the base, shows the torn white tent that served as the base mess hall.