BBC reported that at least 21 people have died after a packed tourist boat capsized in India’s southern Kerala state.

The death toll could rise as rescue efforts are under way on Monday and the vessel is pulled from muddy waters.

Overcrowding caused the double-decker boat to capsize, Abdul Nazar, junior superintendent of police of Malappuram district had told Reuters news agency. The boat was reportedly carrying about 50 people, or double its capacity, when it overturned on Sunday night.

Indian Prime Minister Narendra Modi expressed condolences on Twitter, saying he was “pained by the loss of lives”.

Many passengers were trapped under the boat and the darkness held back rescue efforts, according to local media. The casualties included women and children on school holiday.

At least four people who were taken to hospital are in critical condition, said Kerala’s sports and fisheries minister, V Abdurahiman.

Many of the passengers were not wearing life jackets at the time of the incident, survivors told local media. The exact number of missing passengers was not immediately clear.

Authorities are investigating the cause of the mishap.