The homicide bomber believed to have blown himself up in a U.S. military dining tent near Mosul (search) this week, killing more than 20 people, was probably wearing an Iraqi military uniform, the U.S. military said Thursday.

The top U.S. general in northern Iraq acknowledged that the bomber may have gotten through the vetting process conducted by U.S. and Iraqi authorities to check the backgrounds of Iraqis joining the security services.

Lt. Col. Paul Hastings, spokesman for Task Force Olympia (search) in Mosul, said a general officer will be flying in from headquarters in Baghdad to take over the investigation into how the devastating attack on the base near Mosul was carried out. The FBI is also participating in the probe.

"He'll initiate an investigation ...then we will be in a better position to find out what happened," Hastings said in a telephone interview.

The Ansar al-Sunnah Army (search), the military group that earlier claimed responsibility for the attack, issued a new statement reiterating that it was a homicide bombing.

"God enabled one of your martyr brothers to plunge into God's enemies inside their forts, killing and injuring hundreds," the group said in a statement posted on its Web site Thursday. "We don't know how they can be so stupid that until now they have not figured out the type of the strike that hit them."

The blast Tuesday was the deadliest single attack on a U.S. base, hitting the dining tent at lunchtime and killing 14 U.S. servicemembers, four American civilians, three Iraqi National Guard members, and one "unidentified non-U.S. person." Military officials have said it's not yet known whether that final death was the homicide bomber.

"From preliminary indications of the damage it looks like the guy (the homicide bomber) was wearing an Iraqi military uniform," Hastings said, adding that it seemed like a "vest-type of explosive."

Investigators had still not determined whether the attacker was working on the base or whether he had managed to infiltrate it, Hastings said.

Members of the Iraqi interim government's fledgling security forces routinely operate with U.S. troops in operations against the insurgents. Until now, there have been no reports of serious tensions between the two.

Iraqi government officials said they knew nothing of the report that bomber may have been wearing a uniform. "This was an American declaration, we don't know any thing about it, they did not contact us," said Salih Sarhan, a spokesman for the Iraqi Defense Ministry.

In an interview with CNN, Brig. Gen. Carter F. Ham -- commander of Task Force Olympia, the main U.S. force in northern Iraq -- also reported the bomber was likely wearing an Iraqi uniform and said the attacker may have gotten through the vetting process run by U.S. and Iraqi authorities.

"The vetting process I think is sound, but clearly we have now at least one instance where that was likely not satisfactory. So we have to redouble our efforts there," he said.

Ham said the bomber likely had help, though he did not say whether it was known if the bomber had accomplices in the camp.

"It is very difficult to conceive that this would be the act of a lone individual. It would seem to me reasonable to assume that this was a mission perhaps sometime in the planning, days perhaps," he said.

Amid the investigation, the military is reassessing security at bases across Iraq in light of the bomber's success in apparently slipping into the camp, entering a tent crowded with soldiers eating lunch and detonating his explosives.

The attack's apparent sophistication indicated the bomber probably had inside knowledge of the base's layout and the soldiers' schedule.

Jeremy Redmon, a reporter from the Richmond Times-Dispatch embedded with troops at the Mosul base, said Iraqis working on the base show identification to get in to the base, but once inside move with relative freedom.

At the targeted dining hall "there was no security that I saw," Redmon said in an interview. He said that during the Muslim holy month of Ramadan -- in October and November, when authorities had increased concern of attacks -- civilians were screened as they entered "but that stopped after Ramadan."

Reports earlier Thursday indicated that security had been boosted in at least several U.S. bases and other facilities following the homicide bombing.

Hastings said that armed guards were posted at the entrances and exits from dining halls and other communal areas at his base in northern Mosul.

Early Thursday, hundreds of U.S. troops, Iraqi National Guards and Kurdish militiamen were seen in the streets of Mosul moving around in Bradley Fighting vehicles. In some eastern neighborhoods they searched homes for weapons. One of the city's five bridges over the Tigris River reopened Thursday, after all were blocked off by U.S. troops a day earlier.

Iraqi National Guards manned a checkpoint near another U.S. base, the former palace of Saddam Hussein, stopping passing cars and searching them.