Suspicion has fallen upon Baluch separatists, fighting for more autonomy and a greater share of profits from the region's natural resources.
The separatists in Pakistan's biggest, but poorest province accuse the central government of exploiting its resources without sharing the benefits among its people.
They frequently attack gas pipelines and government and military targets.
Earlier on Tuesday, suspected separatists blew up a gas pipeline on the outskirts of Quetta, cutting supplies to four districts near the city and a power plant.
Supplies might be restored to some areas within 24 hours, officials said.
Pervez Musharraf, the Pakistani president, has promised to develop the province which borders Afghanistan and Iran.
He has also promised an amnesty for separatists who give up, but vowed tough action against those who do not.