The explosion in a US army base in Mosul yesterday in which 22 people were killed apparently was caused by a suicide bomber, the US military said today.
Air Force General Richard Myers, chairman of the Joint Chiefs of Staff, said investigations showed the blast was caused by an "improvised explosive device" worn by an attacker.
The attack at a US base in Mosul killed 22 people, including 13 soldiers, five American civilians, three Iraqi National Guards and an unidentified non-American civilian.
Four employees of the US oil service firm Halliburton were among those killed, the company said.
Iraq's Ansar al-Sunna militant group have claimed responsibility.
FBI and other experts flown in from Baghdad are probing the cause of the lunchtime explosion at a US base mess tent, the deadliest strike on Americans in Iraq.
US forces sealed off entire districts of Mosul today, blocking bridges and raiding homes in a hunt for suspects.
Mosul's governor issued an overnight order on television banning use of the five bridges over the River Tigris and said anyone breaking the order would be shot. Residents said Iraq's third city was a virtual ghost town, with no one in the streets.
The attack has raised fears of a new offensive before next month's election, six weeks after US troops stormed the rebel stronghold of Falluja in a bid to crush the insurgency. Hitherto quiet Mosul has seen near anarchy since.
US President George W Bush last night sent his condolences to the families of the US soldiers killed in the attack. "We send our heartfelt condolences to the loved ones who suffered today. We just want them to know that the mission is a vital mission for peace," he said.
"I'm confident democracy will prevail in Iraq. I know a free Iraq will lead to a more peaceful world," he added. Mr Bush paid a Christmas visit to families of wounded soldiers as a poll showed most Americans now believed the war was not worth fighting.