A homicide bomber blew himself up in front of a high school north of Baghdad on Tuesday, wounding 22 people including teachers and students arriving for the beginning of the school day.

Bystanders and at least one police officer were also wounded in the 8:30 a.m. bombing, according to a policeman who spoke on condition of anonymity because he feared reprisals.

The target of the latest bombing was unclear: The school is next to the provincial governor's office and a municipal building in Baqouba, 35 miles northeast of Baghdad.

Baqouba is the turbulent capital of Diyala province, which has defied a nationwide trend toward lower violence over the past six months. Al Qaeda in Iraq fighters fled to Diyala after Sunni insurgents and clans members joined with American troops to oust them from much of Baghdad and Anbar province to the west.

U.S. commanders credit anti-Al Qaeda fighters from Sunni groups, a six-month cease-fire by a Shiite militia and the dispatch of 30,000 additional U.S. soldiers last year for the reduction in violence. But there has been an uptick in high-profile bombings in recent weeks, suggesting Al Qaeda remains a potent threat.

The Diyala attack followed three homicide attacks in as many days in Sunni Arab areas thought to have been largely rid of Al Qaeda militants.

On Monday, a homicide bomber apparently targeting a senior security official blew himself up inside a funeral tent, killing 18 people in Hajaj, a village about midway along the nearly 20 miles between Saddam Hussein's hometown of Tikrit and the oil hub of Beiji, 155 miles north of Baghdad. But police said the attack bore the hallmarks of Al Qaeda in Iraq.

Witnesses said about 70 people were inside the tent when the attacker set off his explosives soon after entering.

Officials said the target appeared to be Ahmed Abdullah, deputy governor in charge of security for Salahuddin province, of which Tikrit is the capital. He escaped unharmed.

Policemen on duty outside the tent did not search visitors, said a witness, Mukhlis Salim, who lives nearby.

The attack came one day after a teenage homicide bomber targeted U.S.-backed, anti-Al Qaeda fighters near the former insurgent stronghold of Fallujah in Anbar province west of Baghdad. Six people were killed by that blast.

On Saturday, three homicide bombers attacked a police station in Ramadi, Anbar's provincial capital. Guards killed one attacker, but the other two detonated their explosives at the entrance, killing at least five officers.

Meanwhile, a military spokeswoman said a soldier killed over the weekend south of Baghdad was the first American casualty in a roadside bomb attack on the newly introduced, heavily armored MRAP, or Mine-Resistant, Ambush-Protected vehicle.

The hull of the huge armored truck is V-shaped, designed to deflect blasts from roadside bombs, a weapon that has killed more American soldiers than any other tactic used by Sunni insurgents and militia fighters in Iraq.

The soldier who died Saturday was the gunner who sits atop the MRAP vehicle. Three crew members tucked inside the cabin were wounded. The vehicle rolled over after the blast and it was not clear how the gunner died — whether from wounds in the explosion or in the subsequent roll-over.

Maj. Alayne P. Conway, deputy spokeswoman for the 3rd Infantry Division, said the attack and the death were under investigation.

There now are more than 1,500 of the costly vehicles in service in Iraq and the Pentagon is working to get at least 12,000 more into the theater, using $21 billion provided by Congress. MRAPs cost between $500,000 and $1 million, depending on their size and how they are equipped.

The sophisticated vehicles are being built and put into service in a bid to provide soldiers and Marines more protection than is offered by armored Humvees, which have flat bottoms which absorb the shock waves from a blast. The bottom of an MRAP also is 36 inches above the ground, while Humvees sit much closer to the roadway.