At least 12 people were killed and just as many injured on Saturday when a powerful explosion tore through the premises of a private bank in Karachi, the Express Tribune reported.The building, located near Paracha Chowk, was built on a nullah and has a fuel station adjacent to it.Following the explosion, the bank premises collapsed and most of the casualties are reportedly customers and staff present inside the bank, the report said.The cause of the explosion is being investigated and officials say they cannot confirm yet whether it was a gas pipeline or a build-up of methane that took place inside the nullah which led to the explosion."We have no idea whether it was a gas blast. Our first priority is the rescue operation and then we can determine the cause of the explosion," said DIG East Sharjeel Kharal."It might be a sewer gas blast as the bank was constructed on a nullah. It is too early to say whether it was a gas line or a sewer explosion. We are investigating," said the local deputy commissioner.The explosion was heard for miles and shattered windows of nearby buildings and also damaged vehicles parked in its proximity.