At least 15 people were killed and 16 others injured in an explosion on Saturday at a bank branch in Karachi’s industrial area, police said.

A senior police officer said the blast at a branch of Habib Bank Limited, Pakistan’s largest lender, could have been caused by a gas leak, Sarfaraz Nawaz also told reporters that the building appeared to have been constructed over a sewage drain.

“Our explosives teams are at work trying to ascertain the nature of the blast, but apparently the structure was constructed on a drain and gas could be a probable cause,” he said.

In a statement on Twitter, Habib Bank said an explosion had damaged its branch and caused casualties and injuries. “Our sympathies are with the bereaved families,” it added.

A petrol station located next to the bank within the Sindh Industrial Trading Estate and cars parked nearby were badly damaged. Television footage showed the bank’s floor ripped apart.

“So far 15 people have died and 16 were brought injured to hospital,” Karachi’s administrator Murtaza Wahab said in a tweet.

Another senior police officer, Sharjeel Kharal, told reporters it was not immediately clear how many were dead or injured in the explosion as the bank had a skeleton staff on Saturday.