A U.S. military investigation has concluded that a suicide bomber wearing an Iraqi uniform carried out the deadly attack in a dining tent at an American base in Mosul last December, killing 22 people.

Major General David Rodriguez Friday said the bomber apparently entered the base at an unguarded point on its perimeter. He said investigators did not know if the attacker was a member of the Iraqi security forces, or was wearing a stolen or counterfeit uniform.

General Rodriguez said the bomber was believed to be a member of the Ansar al-Sunna terrorist group, which claimed responsibility for the December 21 attack - the deadliest on U.S. forces since the U.S.-led invasion in March, 2003.

The group has also claimed responsibility for several other large profile attacks, including this month's roadside bombing south of Haditha that killed 14 U.S. Marines.