BAGHDAD — Bombs killed more than 50 people in Iraq on Thursday in the worst violence since U.S. combat troops withdrew from urban areas last week, and American forces released five Iranian officials suspected of aiding Shiite insurgents.

U.S. officials said they believe the Iranians, detained in northern Iraq in January 2007, had facilitated attacks on American-led forces but handed them over to the Iraqi government at its request. Iraq's foreign minister, Hoshyar Zebari, said it was a "good initiative" that could encourage dialogue between Washington and Tehran, which are longtime foes.

The Iranian Embassy said it expected to receive the Iranians, described by their government as diplomats. Washington believes they are members of the Quds Force, part of Iran's powerful Revolutionary Guard Corps.

The carnage within Iraqi borders Thursday was a sign that insurgents remain intent on destabilizing Iraq as the United States shifts its focus to the war in Afghanistan. Attacks are down sharply from past years of war and militants have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.

The most lethal attack Thursday was in the northern city of Tal Afar, where women sat in the street amid torn and bloodied bodies in the aftermath of suicide bombings, wailing and beating their chests in grief. Several men crouched and wept into their hands. Others rushed the wounded to ambulances; some used a bed sheet as a makeshift stretcher.

In a statement on his Web site, Iraqi President Jalal Talabani condemned the attacks and said the "forces of evil and terrorism" were trying in vain to demoralize Iraqi security forces and the civilian population.

Some 130,000 U.S. troops remain in Iraq, but they have a much lower profile and are preparing for a complete pullout by the end of 2011. Iraqi attitudes are mixed, with some rejoicing over the absence of American troops in their streets and a new sense of sovereignty, and others worried that extremists will now have more freedom to operate.

"Our security forces are still weak, with poor intelligence," said Saeed Rahim, a government employee in Baghdad. "Deploying more unqualified troops into the streets does not necessarily lead to better results."

The day's violence began at 6:30 a.m., when a suicide bomber in a police uniform and carrying a radio and a pistol knocked on the door of an investigator in the anti-terrorism police force in Tal Afar. When the officer opened the door, the bomber detonated his explosive belt, killing the officer, his wife and son, said Maj. Gen. Khalid al-Hamadani, police chief of the northern Ninevah province.