KARACHI, Pakistan—A powerful gas explosion in a sewage system in Pakistan’s largest city, Karachi, killed at least 12 people and injured 11 others Saturday, police and a health official said. Police spokesman Sohail Jokhio said the blast was apparently caused when something ignited gas that had accumulated in the sewer beneath a local bank building in the Shershah neighborhood of the port city. The powerful blast destroyed the HBL bank building. Video footage aired by local television stations showed the entire structure of the Shershah branch was damaged. The floor was blown out and furniture was mired in sewage from below. The impact of the blast damaged several cars parked nearby and threw debris onto a gas station on the other side of the building. Jokhio said it was not yet clear what ignited the gas but a team of explosives experts had been summoned to investigate. Dr. Sabir Memon …