The U.S. military is investigating how a suspected suicide bomber was able to penetrate an American army base near the northern Iraqi city of Mosul Tuesday killing 22 people.

An al-Qaida linked militant group in Iraq (Army of Ansar al-Sunna) has claimed responsibility for the attack, which also wounded 69 people, many of them American soldiers.

Meanwhile, in the former insurgent stronghold of Fallujah, explosions could be heard (Thursday) as some residents displaced by November's military offensive waited to re-enter the city for the first time.

In other developments, Iraqi authorities have arrested six suspects in connection with Sunday's bombing in Karbala that killed 14 people. And raids south of Baghdad, have netted 44 suspected insurgents in an area with many kidnappings and killings.

Also, an American soldier was killed and two were wounded in a bomb blast in Baghdad.