A boat ferrying more than 100 people home from a party sank in Brazil's Amazon region before dawn Sunday, killing at least 12 and leaving dozens missing, rescue officials said.

The Comandante Sales capsized and swiftly sank in the Solimoes River, one of the Amazon's largest tributaries, Fire Lt. Col. Raimundo Rodrigues da Silva said. The vessel was taking passengers home from a party near the jungle city of Manaus.

It was unclear what kind of boat the Comandante Sales was or how many passengers it was equipped to carry — but most boats that move people across the Amazon region are large wooden ferries, sometimes with two stories. The area has few roads.

Rescue workers recovered the bodies of 10 women and two men while several dozen others were unaccounted for, Silva said, noting that it was unlikely the boat had carried a passenger list. Victims had not been identified.

Investigators were exploring what caused the boat to capsize. It may have been overloaded and many sleeping passengers trapped inside when it went down, Silva said.

Some survivors swam to shore, local media reported, although Silva said authorities did not know how many. Brazil's navy joined about 45 rescue workers, including 15 divers, in searching the area, about 80 kilometers (50 miles) from Manaus, the capital of Amazonas state.

Boat accidents have killed at least two dozen people in Brazil since February, when a two-story wooden ferry carrying more than 100 people collided with a barge loaded with fuel tanks on the Amazon River, killing 16.

In March, a similar boat carrying 12 tourists and a crew of 10 on a wetlands fishing trip sank in the Pantanal area in Mato Grosso state south of the Amazon, killing nine.