According to the national search and rescue agency, Indonesian officials retrieved the dead of 15 passengers and were looking for another 19 individuals who went missing when a tiny ferry overturned during a brief 20-minute ride on Sulawesi island.

Only six of the 40 people on board were known to have survived the Sunday night crash, according to the agency.

“All of the victims have been identified and handed over to their families, while the survivors are now being treated in local hospitals,” said Muhamad Arafah of the search and rescue agency’s local branch.

Photos shared by the rescue agency showed bodies covered in cloth on the floor of the local hospital.

The cause of the sinking, which occurred at about midnight, was still unclear.

The vessel was ferrying people across a distance of just 1 kilometer (less than half a mile) in Muna island, about 200 km (124 miles) south of Kendari, the capital of Southeast Sulawesi province. The journey, between two small towns on either side of Mawasangka Bay, was meant to be just 20 minutes long.

Teams searching for the missing were diving at the location where the vessel sank and patrolling nearby waters in rubber boats, the agency said.

Ferries are a common mode of transport in Indonesia, an archipelago of more than 17,000 islands, and accidents are common as lax safety standards often allow vessels to be overloaded without adequate life-saving equipment.