QUETTA, Pakistan  —  Two gunmen on motorcycles sprayed a truck full of policemen with machine-gun fire in this Pakistani (search) town near the Afghan border Sunday, killing at least 11 and wounding nine, police said.

Most of the police were members of the minority Shiite sect of Islam (search), a Shiite leader in the southwestern town of Quetta said, suggesting the attack may have been the latest by extremist Sunnis.

The policemen were riding in the back of a pickup truck on their way to a training school when they came under attack on Quetta's main thoroughfare, said police senior superintendent Rehmatullah Khan Niazi.

The gunmen fled, and no arrests had been made. Authorities surrounded the bloodstained blue pickup truck.

The motive for the attack was not immediately clear, Niazi said.

But Sardar Musa, head of the local Hazara tribe, said many of the police trainees were members of his ethnic group. Hazaras are mainly Shiite Muslims (search).

"We want the government to arrest the culprits," Musa said.

The killings were the third in just over a week against Shiites in Quetta, capital of southwestern Baluchistan province.

Pakistan's population is 80 percent Sunni Muslim, the majority sect in Islam. Though the two sects mainly get along, small groups of extremist Shiites and Sunnis frequently attack each other.

On Friday, motorcycle gunmen in Quetta shot and killed a Shiite Muslim prayer leader.

And on May 31, two gunmen riding motorcycles ambushed a car carrying Ghulam Nabi, a prominent local Shiite, wounding him and killing his son.