The U.S. military in Iraq has begun an investigation into Tuesday's blast at a base in Mosul, as well as a search for those responsible for an explosion at a mess hall tent that killed 22 people, among them 13 American soldiers.  It was one of the deadliest attacks on American troops since the war began. 

U.S. military investigators are looking into whether it was a bomb smuggled onto the base, or a rocket fired at the tent where servicemen, both American and Iraqi, were having a midday meal.

"It is still under investigation as to the cause," said U.S. Army Captain Darren Luke in Baghdad. "They do have individuals on the ground trying to figure out what has happened and when we do get information on that, we will release it."

An Iraqi militant group called Ansar al-Sunna claimed responsibility for Tuesday's explosion. 

With the hunt for the perpetrators under way, a U.S. military spokesman in Mosul says forces are now involved in offensive operations with specific objectives. 

Witnesses say American troops backed by Iraqi forces have blocked off entire neighborhoods.  Journalists in Mosul report a heavier military presence in the city and stepped up aircraft on patrol overhead.

The more than 50 Americans injured in the attack have begun arriving for treatment at a U.S. military hospital in Landstuhl, Germany.  

Tuesday's blast in Mosul amounted to the second-highest U.S. troop death toll from an insurgent attack since the start of the Iraq war last year.  

Mosul was also the scene of the deadliest rebel-related incident when two U.S. military helicopters came under insurgent fire and collided in the city in November of last year.