NEW DELHI (AP) — Eleven people died and four more were hospitalized after a gas leak Sunday in northern India’s Punjab state, local media reported.

The incident occurred at an industrial area in Ludhiana city, but the source of the gas leak is still unclear, police told the Press Trust of India news agency.

India’s National Disaster Response Force sealed off the densely populated area and evacuated residents.

Authorities were trying to ascertain the type of gas in the leak, PTI quoted disaster response officials as saying.

Ludhiana Deputy Commissioner Surabhi Malik told the news agency it was possible that the gas may have spread from manholes.

“We are going to collect samples from manholes. It is quite likely that some chemical reacted with methane in manholes,” she said.

The state’s chief minister, Bhagwant Mann, said the leak came from a factory but didn’t provide more details in a tweet. “All possible help is being provided,” he wrote.