A massive car bomb exploded near the police chief's headquarters in the southwestern Pakistani city of Quetta on Thursday, killing two police and wounding four others, police said. The bomber also died.

Officials initially suspected it was a suicide attack, but Baluchistan provincial police chief Chaudhry Mohammed Yaqoob, who was inside the headquarters and escaped unharmed, told reporters that the bomb had apparently gone off prematurely as the attacker was driving to his intended target.

There was no immediate claim of responsibility for the blast. Asked if Taliban militants or the government of neighboring Afghanistan could be involved, Yaqoob said, "We cannot rule out any possibility right now."

The bombing followed recent police arrests of Taliban-linked militants operating in and around Quetta, a city near the Afghan border that boasts a large Afghan refugee population.

Baluchistan is Pakistan's largest and most impoverished province, but is also rich in natural resources, including gas. Baluch tribesmen have been waging years of attacks against government forces in a bid to win more control of resources extracted in the province.

Click here to go FOXNews.com's Asia Center.

Although Yaqoob said the car blew up as it traveled down a road opposite his office, the destruction caused by the blast suggested it had gone off in a parking lot nearby. The explosion left a shallow crater about 5 feet wide there and scattered body parts and wreckage from the attacker's car as far as 300 feet.

Police initially said one policeman and a civilian were killed, but Yaqoob said the dead were both policemen. The wounded included three police and a civilian.

Manzoor Ahmed, a policeman wounded in the blast, said he had been riding in a pickup truck when the bomb went off to his left where the parking lot is located. As he was taken to a hospital for treatment to his shoulder, he said he saw two bodies on the ground near the blast site.

Police blocked off the street as investigators sifted through debris for clues. A mobile phone and the bomber's identity card were found in the pocket of his shirt, Yaqoob said, but did not divulge further details.

Police also defused a 13 pound bomb rigged to a motorcycle parked near a police station in downtown Quetta about two hours before the bombing, said Abdul Wahid Qazi, a senior police officer.

It was unclear if the explosion was linked to Monday's Pakistani air raid on a religious school in the northwestern Bajur district — hundreds of miles to the north — which killed 80 suspected militants.

Pro-Taliban tribesmen have since threatened to launch suicide attacks against Pakistani forces.