The brigadier general who investigated abuse at Abu Ghraib prison has launched a wide-ranging probe into security lapses that allowed a suicide bomber to penetrate into a packed mess hall at a base near Mosul and kill more than 20 people, authorities said Friday.

Brig. Gen. Richard P. Formica (search) leads the investigative team, which started work as Defense Secretary Donald H. Rumsfeld (search) flew into Mosul to pay a surprise Christmas Eve visit to the wounded soldiers at the base, part of a tour that also took him to Tikrit, Fallujah and Baghdad.

"Now we have a pretty good idea that it was a suicide bomber," said Lt. Col. Paul Hastings, a spokesman at the Mosul base. Formica "is going to investigate into the how's — how did that happen?"

Hastings said the Mosul investigation will be "conducted quickly and thoroughly" but that there was no deadline for its conclusions.

The Mosul blast killed 22 people, most of them American soldiers and civilians — the deadliest single attack on a U.S. base in Iraq. The success of the attack — claimed by the radical Islamic group known as the Ansar al-Sunnah Army — prompted calls for a thorough investigation on ways to prevent future suicide attacks.

The suicide bomber believed to have carried out the attack was probably wearing an Iraqi military uniform, the U.S. military has said.

On Friday, around 4,000 displaced citizens returned to inspect their homes in Fallujah, which was devastated in a U.S. offensive last month that broke insurgent control over the city.

Iraq's interim security minister Kassim Daoud said people were insisting on returning despite clashes that have continued in the city since the offensive ended — including heavy fighting on Thursday that killed three U.S. Marines.

A posting Friday on an Islamic Web site made a rare admission of significant casualties among insurgents in Thursday's clash, saying 24 were killed. Nineteen were said to be non-Iraqi Arabs from Saudi Arabia, Kuwait, Bahrain, Egypt, Syria and Jordan. The rest were said to be Iraqis.

Such postings often refer to large U.S. casualties. References to dead among the rebels aren't unusual, but usually the numbers given are small.

Most of Fallujah's approximately 250,000 people fled before the U.S. assault, and repatriating them is a key step in the attempt to restore stability in the city ahead of Jan. 30 elections. Much of Fallujah remains uninhabitable because of destroyed homes, unexploded ordinance on the streets, lack of water or basic supplies and commodities.

Many of those who arrived Friday were shocked and angry. Some said they would rather remain in makeshift camps outside the town than return to their bombed out homes.

"I no longer have my home," said a man who identified himself only as Abdul-Rahman. "I prefer the camp to returning to Fallujah in this terrible way. I don't know when am I going to be bombed or killed."

Some of the returnees were turned away by the authorities for not carrying identification papers proving they were Fallujah residents. The formal return started Thursday when nearly 1,000 residents, mostly men, entered the city.

Rumsfeld met with Iraqi interim president Ghazi al-Yawer (search) while in Baghdad, and the two reiterated that the elections should be held in January.

In other developments Friday, U.S. soldiers opened fire on car carrying a family in Baghdad, killing a young girl and injuring her mother and brother, an APTN report said. The circumstances of the incident on the treacherous Airport Road, the scene of frequent bomb attacks against American troops, were unknown.

U.S. troops and insurgents clashed in the city center of Samarra, 60 miles north of Baghdad, leaving four civilians wounded, police Maj. Saadoun Matroud said.

Gunmen kidnapped Iraqi National Guards Col. Saadi Aftan Hammoud while on his way from Baghdad to western city of Ramadi, the police said. Four other guard members who were with Hammoud, a commander in Ramadi, were allowed to continue on their way, the officer said.