A guerrilla attack on a tented dining hall at a US base in northern Iraq killed 18 Americans and four Iraqis in the deadliest strike on US forces since last year's war to oust Saddam Hussein.
US officials initially said a number of rocket and mortar rounds were fired at the base in the northern city of Mosul on Tuesday, but a militant group claimed a suicide bomber was behind the attack, in which a further 72 people were wounded.
"A suicide bomber has not been ruled out," a US Army official said later in Washington.
The military in Baghdad said 14 US soldiers, four US civilians and four Iraqi security force members were killed.
Fifty-one of the wounded were US military personnel. Of the 72 hurt, 43 were still being treated.
A spokesman in Mosul said it could take days before a clear picture emerged of what happened and who was killed.
"There's some tedious forensic work to be done. It could be a couple of days," said Captain Phil Ludvigson, who said it was not clear what caused the explosion.
Mosul residents said US forces sealed off areas of the city yesterday, including bridges crossing the Tigris river, and carried out raids in a hunt for suspects behind the attack.
Two French journalists, held hostage for four months were freed, ending a saga that had embarrassed the Paris government.
French Prime Minister Jean-Pierre Raffarin said the two journalists would be home for Christmas. A military plane was due to arrive in Baghdad from Paris to take them home.
Meanwhile, Iran's most senior dissident cleric urged Iraqis to vote and said their planned Jan. 30 elections could mean peace and independence.
"Under the supervision of the top clerics, all Iraqis should vote to form a powerful government through free, nationwide and broad-based elections," Ayatollah Hossein Ali Montazeri said in a letter to leading Iraqi Shiite cleric Grand Ayatollah Ali al-Sistani.