The United States handed over its last detainee in Iraq to Iraqi authorities on Friday, the White House said, after months of failed efforts to convince Baghdad to extradite him because of his suspected role in the killing of Americans.

White House National Security Council spokesman Tommy Vietor told Reuters the United States had received assurances that Ali Mussa Daqduq, a suspected Hezbollah operative, would be tried for his crimes.

But initial reaction from Congress was fierce. Lawmakers fear that Iraq will be unable or unwilling to hold Daqduq for long and several used words like "disgraceful" to describe the handover. Senator Saxby Chambliss accused the Obama administration of "completely abdicating its responsibility to hold on to deadly terrorists."

Daqduq is accused of orchestrating a 2007 kidnapping that resulted in the killing of five U.S. military personnel and his fate became a source of growing concern in Washington this year as the U.S. military prepared for an end-2011 withdrawal.

Indeed, Daqduq was the only detainee that the U.S. military held onto when it transferred everyone else to Iraqi authorities in November. U.S. officials had hoped for a last minute deal which failed to materialize.

"As of this morning, he's been transferred to Iraqi custody," Vietor said.

"We've worked this at the highest levels of the U.S. and Iraqi governments and we continue to discuss with the Iraqis the best way to ensure that he faces justice," he said.

It was unclear whether the United States might later try to win custody of Daqduq.

Senator John McCain was among the harshest critics of the handover. He and three other lawmakers warned Daqduq might be released from Iraqi custody for political reasons and "return to the fight" against the United States and its allies.

"We are deeply concerned that Daqduq will never have to answer for his involvement in killing U.S. citizens," McCain said in a statement also signed by Senate Republican leader Mitch McConnell and two other senators.

The transfer of Daqduq, who was born in Lebanon, to Iraqi custody represents one of the risks the United States faces in ending the nearly 9-year-old war, with all American troops due out of the country this month. A senior Iraqi officer last month told Reuters about efforts by individuals in Lebanon and Iran to win custody of Daqduq.

"If the administration is truly serious about this case, they'll try him for war crimes in a military tribunal, not increase the chances that he will end up sipping tea in Tehran," said Senator John Cornyn.

U.S. Defense Secretary Leon Panetta also acknowledged that Iraq retaining custody was not the ideal solution, telling Congress last month that Daqduq would face "better justice" in the United States.

Panetta was in Baghdad on Thursday for the ceremony formally ending the Iraq war, in which almost 4,500 U.S. troops and tens of thousands of Iraqis were killed.

U.S. military officials had hoped to leave several thousand American military trainers behind to assist Iraq but Baghdad and Washington failed to reach an agreement. The troop deal fell apart over U.S. demands that Iraq's parliament approve legal guarantees to protect remaining U.S. forces from prosecution.

In the case of Daqduq, sources told Reuters that Baghdad was unwilling to cede custody to the United States of an individual it saw as a prisoner of Iraq.

Violence in Iraq has ebbed since the bloodier days of sectarian slaughter when suicide bombers and hit squads claimed hundreds of victims a day as the country descended into tit-for-tat killings between the Sunni and Shi'ite communities.

In 2006 alone, 17,800 Iraqi military and civilians were killed in violence.

Iraqi security forces are generally seen as capable of containing the remaining Sunni Islamist insurgency and the rival Shi'ite militias that U.S. officials say are backed by Iran.

But attacks now target local government offices and security forces in an attempt to show the authorities are not in control.-Reuters