More than 40 people died in bombings in Iraq on Thursday in the worst violence since United States combat troops withdrew from urban areas at the end of last month.

Even though attacks are down sharply from past years of war, the carnage was a reminder that insurgents remain intent on destabilizing Iraq as the United States shifts its focus to the war in Afghanistan. Militants have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.

Some 130,000 U.S. troops remain in Iraq, but they have a much lower profile in line with a security agreement with Iraq that mandates their 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.

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 the northern city of Tal Afar. When the officer opened the door, the bomber detonated his explosive belt, killing the man, 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 34 people and injured 70. A day earlier, 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 the region’s long-term stability.

Insurgents also struck Baghdad on Thursday morning, detonating roadside bombs that killed seven people. Six died and 31 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. Explosives experts defused a third bomb in the area.

Hassan Abdullah, a 32-year-old vegetable salesman, said he heard the first blast and went to see what was happening when a second bomb hidden in trash about 100 meters away exploded. He said he fell to the ground and was taken to a hospital with hand and leg injuries.

In the Karrada district of central Baghdad, one civilian was killed and five people were injured in a roadside 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. Three of the injured were guards of the governor, who was unharmed.

Five people were injured in bombings in other parts of Baghdad.

Also Thursday, U.S. forces released five Iranian officials detained in January 2007 in northern Iraq on suspicion of aiding local Shiite militants, Iranian and Iraqi officials said. Iraq’s foreign minister, Hoshyar Zebari, predicted the release would help improve dialogue between Iran and the United States, which are longtime adversaries.

The U.S. military did not comment on the Iranians, saying the release and transfer of detainees to Iraqi authorities was a private matter that occurs regularly in line with a security agreement. Separately, it said it was investigating the death of a U.S. soldier who had been found “unresponsive” on a military base.