The National Disaster Response Force is extracting gas samples from sewers in the area in an effort to identify the gas.

At least 11 people were killed and nine were hospitalised following a gas leak in the northern Indian state of Punjab on Sunday.

Women and children were among those who died in the port city of Ludhiana, where several other people were found unconscious in their homes.

A team from the National Disaster Response Force was at the site, in the Giaspura area of Ludhiana, along with a team of experts to determine the cause and source of the leak, said Rajinder Pal Kaur Chhina, a member of the local legislature.

The force is also extracting gas samples from sewers in the area in an effort to identify the gas. One official suggested that 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,” PTI news agency quoted Ludhiana Deputy Commissioner Surabhi Malik as saying.

Officials had sealed off a section of the area and were evacuating residents after many reported breathing difficulties.

All the victims were found near three businesses in the Giaspura industrial district – a dairy store, a grocery store, and a clinic. The area is home to clothing factories and the people who work there are hundreds of migrant workers from the states of Uttar Pradesh and Bihar.

The deaths were mainly caused by suffocation, said Dr. Hatinder Kaur of the Civil Hospital of Ludhiana, where the victims were taken and the injured were still being treated. Among the dead were two children, local news media reported.

Chhina said the incident had happened near a milk shop. “People who came to buy milk in the morning fell unconscious outside,” she said. ‘We cannot say for sure where the leak began.”

Punjab Chief Minister Bhagwant Mann said on Twitter the leak was from a factory but did not provide further details.

Industrial gas leaks are not uncommon in India. A gas leak three years ago at a chemical plant in the Andhra Pradesh city of Visakhapatnam left at least 12 people dead.

In 1984, one of the world’s worst industrial disasters occurred in the country in 1984, when a gas leak at a pesticide plant in the central city of Bhopal killed thousands of people.