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.

As people gathered in the aftermath, another suicide bomber detonated his explosives belt, al-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 about a "big security vacuum" in the north and said Kurdish forces, known as peshmerga, should withdraw from some areas and allow Iraqi army units to deploy. Tensions between Iraqi Arabs and Kurds, who run a virtual mini-state in part of northern Iraq, are considered a major threat to long-term stability.

Factions are maneuvering for control of Kirkuk, a disputed northern city in an oil-rich area that is seen as a flash point for conflict. Police there said a civilian bystander died in a bomb attack on a police patrol on Thursday.

Insurgents also struck Baghdad on Thursday morning, detonating bombs that killed 18 people and injured dozens. Eight of them died and 30 were injured in coordinated 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.

Hassan Abdullah, a vegetable salesman, said he heard the first blast and went to see what was happening when a second bomb hidden in trash about 100 yards away exploded. He was taken to a hospital with hand and leg injuries.

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 on condition of anonymity because he was not authorized to speak to the media. The governor was unharmed.

In Washington D.C., State Department spokesman Ian Kelly said the U.S. had to release the Iranians under a U.S.-Iraqi security agreement that took effect in January. Kelly said the release was not part of a deal or prisoner exchange with Tehran.

He said Iraq has issued arrest warrants for all non-Iraqi detainees held by American forces and asked the U.S. to transfer them to the Iraqi government's custody.

Kelly described the five Iranians as being "associated with" the Quds force. Kelly said the possibility of the five creating security problems in Iraq was "a big concern."

A senior Iraqi government official said on condition of anonymity that the Americans had advised Iraqi counterparts that the Iranians should leave the country.

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