The attack was one of the deadliest on Iraqi security forces in several weeks. No group took immediate responsibility, but the episode suggested that Sunni Arab guerrillas, who as recently as last spring controlled Baquba, northeast of Baghdad, continue to be able to carry out devastating attacks.  American military officials said they had largely cleared Baquba of militants during operations this summer, when a large force of soldiers swept through the city. But it appears that despite those efforts the city remains unstable, as does much of the rest of the province, where sectarian killings, bombs and kidnappings occur daily.  During the American and Iraqi offensive over the summer, many of the insurgents were able to flee north before the soldiers arrived, American officers said. Some Iraqis have expressed fears that when United States forces reduce their presence in Baquba, the militants will simply return.  Monday’s bombing in Baquba appeared to be part of a coordinated attack on the police force in the provincial capital. At about the same time as that attack, another suicide bomber attempted to strike the police station in Hibhib, on the northern side of the city, according to an American military official in the city. The attack failed because a policeman shot the suicide bomber. However, the coordinated assault suggests that the extremists are active again in the capital.  Diyala has become one of the most lethal places in Iraq for American soldiers, with 110 troops killed so far this year, according to Icasualties.org, an independent Web site that tracks deaths in Iraq. That figure is more than the total number of American troops killed in the province over the past four years. Diyala ranks third after Anbar Province and Baghdad in American casualties.  In Baghdad, the kidnapping of 10 tribal sheiks ended Monday when Interior Ministry commandos obtained the release of nine of them. One of the sheiks was killed shortly after he was kidnapped. The sheiks — three Sunnis and seven Shiites — had just met with officials from the office of Prime Minister Nuri Kamal al-Maliki when they were abducted.  The American military said in a statement that the kidnappers were believed to be rogue elements of the Mahdi Army, the Shiite militia affiliated with the anti-American cleric Moktada al-Sadr. However, at Mr. Sadr’s request, many in the militia have laid down their weapons. The American military says those who have not done so are primarily criminals.  In Karbala, a center of Shiite worship and pilgrimage south of Baghdad, American forces transferred security responsibility to Iraqi forces on Monday. Karbala is the eighth province in which the American military has handed security responsibility to the Iraqis. The city and surrounding province are for the most part calm, but in late August there were clashes between Iraqi forces guarding the holy shrines and militiamen who said they were loyal to Mr. Sadr.  The other provinces that have responsibility for their own security are the three in Iraqi Kurdistan and four southern provinces — Najaf, Dhi Qar, Maysan and Muthanna.  Richard A. Oppel Jr. contributed reporting from Baghdad, and Iraqi employees of The New York Times from Diyala, Karbala and Hilla.  A version of this article appears in print on , on Page A10 of the New York edition with the headline: Suicide Bomber on Bike Kills 29 Iraqi Policemen. Order Reprints| Today's Paper|Subscribe
The bomber also wounded 19 people, including seven policemen who were severely injured and a woman and her baby, the authorities said.