Jakarta, Indonesia (CNN) -- At least 26 people died Sunday when a ferry sank off the Indonesian island of Sumatra, and an unknown number of people were missing, officials said.
There were 250 survivors from the sinking, which occurred during bad weather, according to the ministry of transportation. Strong winds and waves hampered rescue efforts after the ferry sank off Riau province.
The ferry carries passengers between Riau and Batam island.
Bambang Ervan, spokesman for the ministry of transportation, said authorities were investigating because the number of passengers did not match those on the ferry's manifest.
The ministry said earlier that an estimated 260 people were aboard, while the manifest showed 213 on board, including 15 children, plus 12 crew members.
Overcrowding, however, is common on such ferries.
In recent years, hundreds of people have died in ferry accidents on the boats that connect Indonesia's islands because of overcrowding and the fact that safety regulations are poorly enforced.
Before they were rescued, dozens of passengers struggled in rough waters in life jackets, as bad weather forced emergency workers to wait for a bigger rescue boat, Bambang said earlier.
Weather was suspected in the sinking, one of two in the area.
Another ferry sank on its way from Batam. All those passengers were rescued, said Sunaryo, directorate general of sea transport, who only uses one name.