LUCKNOW, India (AP) — A gas leak killed seven people, including three children, at a carpet-weaving factory in northern India on Thursday, a government official said.

Magistrate Suresh Kumar said the gas apparently leaked from a tanker that came to deliver acid used for washing woven carpets.

Five of the seven killed were members of one family, including one woman and three children, he said. It wasn’t clear how many worked there or if some were visiting the factory.

Hydrofluoric acid and ammonia solution are generally used for cleaning carpets.

The factory is in Sitapur, a town 80 kilometers (50 miles) southeast of Lucknow, the capital of Uttar Pradesh state.

Uttar Pradesh is known for its hand-knotted carpet weaving industry.

Such accidents are common in India because of poor safety standards in many factories.