PESHAWAR, Pakistan >> An explosion ripped through a minibus traveling in a militant-infested area of northwestern Pakistan on Monday, killing at least 15 people, police said.

It was unclear whether the blast, which completely destroyed the bus, was caused by a bomb or by the gas cylinder used to power the vehicle, said local police chief Islamuddin Khan.

The bus was traveling between the cities of Hangu and Kohat, which are located very close to Pakistan’s lawless tribal region. Islamist militants frequently carry out attacks in the area against both civilians and security forces.

Local television footage showed the twisted carcass of the bus laying beside the road with little left except its wheels and undercarriage. Another bus nearby was flipped over on its side with its windows blown out and blood splattered across the outside.