A toxic gas leak killed five people and sickened dozens of others in a coastal residential area in Pakistan’s port city of Karachi, police said Monday.

The source of the leak, which occurred on Sunday night, and the type of gas that had leaked were not immediately known. There was no suspicion of sabotage.

Authorities said they became aware of the incident when people in the Karachi neighborhood of Kamari, which is close to the main port where incoming ships anchor, suddenly started rushing to the area’s hospitals with severe breathing problems overnight, police chief Adil Malik told reporters. On Monday, officials were still trying to determine where the gas leak started and what caused it.

Karachi is the capital of southern Sindh province, Pakistan’s largest city and also the country’s chief commercial hub. It also has oil refineries nearby.