BAGHDAD — A suicide bomber on a bicycle blew himself up today in a crowd of police recruits, killing at least 29 people, police and hospital officials said. Separately, a group of kidnapped Sunni and Shiite sheiks were freed, a government spokesman said.

Police and relatives have identified the tribal leaders abducted in Baghdad as seven Shiites and three Sunnis aligned against Al Qaeda who were on their way home to Diyala province — the same region where today’s bombing took place — after attending a meeting with the Shiite-dominated government’s adviser for tribal affairs to discuss coordinating efforts against the terror group.

Police said one of the Sunnis in the group — seized from their cars in a predominantly Shiite neighborhood — was shot to death, but a Defense Ministry spokesman, Major General Mohammed al-Askari, said the rest were freed today. He declined to specify how many or give more details.

The American military, citing intelligence sources, said today that a rogue Shiite militia leader was responsible for the abduction, identifying him as a former brigade commander in the Mahdi Army militia, Arkan Hasnawi.

The leader of the Mahdi Army, Muqtada al-Sadr, in August ordered his fighters to lay down their arms for up to six months, but thousands of followers dissatisfied with being taken out of the fight have broken off to form their own groups that the military says are being funded and armed by Iran to foment violence. Iran denies the allegations.

The military said Mr. Hasnawi’s actions clearly demonstrate that he has violated the cease-fire order and “joined forces with Iranian-supported special groups that are rejecting Muqtada al-Sadr’s direction to embrace fellow Iraqis.”

A member of the Shiite Ambagyah tribe based east of Baqouba said seven of its members were among those abducted.

The kidnappers had offered to release the Shiites but the sheiks refused to leave without their two remaining Sunni colleagues because they feared it “would create more violence and revenge operations,” the tribal spokesman said earlier today, declining to be identified because of the sensitivity of the situation.

The sheiks were returning to Diyala province after attending a meeting with the Shiite-dominated government’s adviser for tribal affairs to discuss coordinating efforts against Al Qaeda in Iraq when they were seized, police and a relative said.

The police recruits in Baqouba were waiting to be allowed inside the camp for the day’s training when the suicide bomber blew himself up in their midst, according to a police officer who spoke on condition of anonymity because he was not authorized to speak to the media.

A 22-year-old Sunni man from Baqouba’s central Tahrir area said he was among a group of some 60 recruits when the blast struck.

Akram Salman said it must have been an inside job because the suicide bomber apparently penetrated heavy security surrounding the police camp without being searched.

He said police failed to stop the bomber when he changed course suddenly from the main road toward the recruits.

“The police are infiltrated. Many people join the police but they have affiliations with Al Qaeda. These infiltrators made it easy for the bomber to attack us,” he said. “There are two main checkpoints on the main road leading to the camp, it would be impossible for a man on a bicycle to pass without being properly searched.”

“Al Qaeda has threatened us before and prevented us from joining the police,” he said. “They slaughtered many policemen, burned their houses, killed their families, and blew up their headquarters. Now, when the people have defeated Al Qaeda and cooperated with the government, Al Qaeda staged this operation to show their presence and to give a message that they are still in control.”

A doctor at the Baqouba general hospital, Mohammed al-Kirrawi, said most of the victims were struck by iron balls packed with the explosives to achieve maximum casualties. He said the hospital lacked the necessary equipment to save many of the wounded.

The attack bore the hallmarks of Al Qaeda in Iraq, whose militants have repeatedly targeted police and army recruits to discourage Iraqis from joining the country’s nascent security forces. Police and hospital officials, giving the casualty toll, also said 19 people were wounded.

Baqouba, 35 miles northeast of Baghdad, is the capital of Diyala province, where hundreds of Sunni Arab tribesmen and insurgents have in recent months joined the American and Iraqi forces in the fight against Al Qaeda. The American military has been trying to extend the strategy to Shiite tribes in a bid to curb Shiite militia violence.

A parked car bomb also exploded near a market in Siniyah, just west of Beiji, an oil hub 155 miles north of Baghdad.

Police said the bomb apparently was targeting a police patrol but missed its target, killing four members of a family who were heading to the market to do their morning shopping and wounding 13 other people.

Another parked car bomb struck a bus station in the western Baghdad neighborhood of Baiyaa, killing at least one civilian and wounding six, police said.

In southern Iraq, meanwhile, the American military turned over security responsibilities to Iraqi authorities in the mainly Shiite province of Karbala, the eighth of the nation’s 18 provinces to revert to Iraqi control.

Prime Minister al-Maliki said the southern province of Basra’s security file would be transferred to the Iraqis in mid-December. The British-led forces overseeing the area already have begun drawing down and pulled back from the center of the provincial capital to the airport on the outskirts.

“This is the proof of the strong will and resolve of the good citizens of this nation,” Mr. al-Maliki said at the handover ceremony in Karbala, 80 kilometers (50 miles) south of Baghdad. “The reconstruction of Iraq does not hinge on security alone, but security is the key to everything.”