"An individual in an Iraqi military uniform, possibly with a vest-worn explosive device, was inside the facility and detonated the facility, causing this tragedy," Brig. Gen. Carter F. Ham — commander of Task Force Olympia, the main U.S. force in northern Iraq — told CNN.
The explosion occurred Tuesday in a base near the northern city of Mosul. Most of the 22 people killed were American military personnel and civilians – the deadliest attack U.S. forces have suffered on one of their bases.
Meanwhile, fighting erupted in Fallujah as residents returned to their devastated city for the first time since a bloody, two-week U.S. military offensive in November that wrested the city from the control of insurgents.
At least three Marines were killed in the area as U.S. forces battled insurgents on Thursday, with warplanes dropping bombs and tanks shelling suspected guerrilla positions in the heaviest fighting in weeks.
In other recent developments:
The new fighting in Fallujah highlighted that the city is far from tamed. Since the offensive, Fallujah has seen sporadic clashes between U.S. troops and pockets of insurgents.
F-18 fighter-bombers were seen striking at targets in the city's outskirts Thursday, and tank and artillery fire was also heard.
American commanders have hailed the November offensive in Fallujah as a major tactical victory. But since then, violence elsewhere in Iraq has only escalated, after many guerrillas apparently slipped out of Fallujah to operate in central and northern Iraq.
Tens of thousands of residents who fled Fallujah have been crowded into camps set up in the region or living with relatives in Baghdad or elsewhere, eager to return to their homes.
Most of Fallujah's approximately 250,000 people fled before the assault.
Authorities had planned to allow the return of 2,000 residents on Thursday, but by afternoon, only about 200 actually made the trip, according to U.S. officials.
Lt. Col. Kevin Hansen, the Fallujah operations officer with the Marines' 4th Civil Affairs unit, said residents may not be aware of the return, and that more may come on Friday after announcements during weekly prayers at mosques.
"Most of them get their information from the mosques so we think that tomorrow they'll get the word out more," he said.
At a checkpoint into the city, cars were lined up, and returnees showed documents to police and pulled out luggage for search. There was some confusion over who was allowed in and when.
"We traveled hundreds of kilometres (miles) to get to the city," said one man, Abu Omar al-Duleimi. "When we arrived, there was no timetable for our return. And they told us that only a small group could enter the city while others were not allowed."
Once inside, returnees found neighborhoods ravaged. "This is all that's left of my property," one man said, waving a dusty blanket. In footage by Associated Press Television News, the corpse of an elderly woman was visible in one destroyed house, lying face down in her black robe. It was not clear how long ago she was killed.
In the wake of the blast in Mosul, U.S. commanders are re-examining security procedures for every American base in the country, trying to figure out how a bomber got into their midst, reports CBS News Correspondent Kimberly Dozier.
They are also hinting there are some major operations in the offing aimed at the enemies who cost so many American lives this week. In the triangle of death south of Baghdad, U.S. troops have already carried out several operations, arresting 44 suspects Wednesday.
General Richard Myers, chairman of the Joint Chiefs of Staff, isn't saying whether authorities think the bomber worked at the base, or breached security.
But military investigators reportedly suspect that the bomber might have been an Iraqi worker who is now unaccounted for.
A radical Sunni Muslim group, the Ansar al-Sunnah Army, said it carried out the suicide bombing at the base.
The Mosul base is surrounded by blast walls and barbed wire and watched by U.S. troops who search every person going in and check his identity.
A contingent of FBI bomb technicians has been deployed to help the military investigate the bombing, said an FBI official on condition of anonymity. The Baghdad-based FBI team will help identify the type of explosive and components used, which could provide forensic links to previous Iraq bombings.
The apparent sophistication of Tuesday's operation indicated the attacker probably had inside knowledge of the base's layout and the soldiers' schedule. The blast came at lunchtime.
"We always have force protection keeping their eyes out," Lt. Col. Paul Hastings, spokesman for Task Force Olympia, the main force that controls northern Iraq, said Thursday. "For somebody that wants to take his life and kill himself, its very difficult to stop those people."