U.S. officials said they believed the Iranians, detained in northern Iraq in January 2007, had facilitated attacks on American-led forces, but they handed them over to the Iraqi government at its request because of an obligation to do so under a U.S.-Iraqi security agreement.

The State Department said it was concerned that their release could present a security threat to U.S. troops in Iraq.

Iraq's foreign minister, Hoshyar Zebari, called the release a "good initiative" that could encourage dialogue between longtime foes Washington and Tehran.

Iranian Embassy spokesman Amir Arshadi said Iraq had transferred the Iranians, whom their government describes as diplomats, to the embassy. Washington believes that they are associated with the Quds Force, part of Iran's powerful Revolutionary Guard Corps, and that they trained Iraqi extremists.

The carnage within Iraqi borders yesterday 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, and extremists have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.

Yesterday's most lethal attack 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 wept into their hands. Others rushed the wounded to ambulances; some used a bed sheet as a makeshift stretcher.

Iraqi President Jalal Talabani condemned the attacks and said on his Web site that the "forces of evil and terrorism" were trying in vain to demoralize Iraqi security forces and civilians.

About 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.

Iraqis' attitudes are mixed. Some rejoice over the absence of U.S. troops in their streets and a new sense of sovereignty; others worry that extremists will now have more freedom to operate.

"Our security forces are still weak, with poor intelligence," Saeed Rahim, a government employee in Baghdad, said.

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 antiterrorism 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.

As people gathered in the aftermath, another suicide bomber detonated his belt, Hamadani said. The coordinated attack killed a total of 38 people and injured 66.

Army Brig. Abdul-Rahman Abu Raghef said the first suicide bomber was a local resident who had been jailed for one year on suspicion of terrorism but was released in an amnesty in June.

A day earlier, car bombs in two Shiite villages near Mosul, another northern Iraqi city, killed 16 civilians and injured more than two dozen.

Haneen Qaddo, a lawmaker representing Shiites in the Mosul region, complained of a "big security vacuum" in the north and said that Kurdish forces should withdraw from some areas and allow Iraqi army units to deploy.

Insurgents in Baghdad detonated bombs that killed 18 people and injured dozens. Eight of them died and 30 were injured in blasts near an outdoor market in the Shiite district of Sadr City, said Maj. Gen. Qassim al-Mousawi, spokesman for the city's operations command center.

In the Karrada district of central Baghdad, one civilian died in a bomb attack on the convoy of Central Bank Gov. Sinan al-Shibibi, a police officer said. The governor was unharmed.

Also yesterday, the U.S. military said it was investigating the death of a U.S. soldier who had been found "unresponsive" on a military base.