Baghdad, Iraq ? The suicide bomber believed to have blown himself up this week in a U.S. military dining tent near Mosul, 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 the bomber might 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 in Mosul, said a general officer would 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 also is 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, the military group that earlier claimed responsibility for the attack, issued a new statement reiterating that it was a suicide 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 Thursday on its Web site. “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.”

Tuesday’s blast 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 bomber.

“From preliminary indications of the damage it looks like the guy (the suicide bomber) was wearing an Iraqi military uniform,” Hastings said.

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