Most victims were members of a single family that chartered the plane to travel to a birthday party, authorities said. Seven children died.
"It was all very fast. The plane sank very fast," 21-year-old survivor Brenda Moraes told GloboNews TV. "We only had a chance because we were sitting in the last rows and could open the emergency door."
Divers recovered two dozen bodies from the twin turboprop plane that plunged into the Manacapuru river in a heavy rainstorm yesterday afternoon, firefighter Major Jair Ruas Braga said.
Authorities said they would investigate whether the plane, with a capacity of 21, had too many passengers.
Firefighters stood hip-deep in the river yesterday examining the plane, which was largely submerged except for the tail and the emergency exit door.
Nearly 20 people on board were family members, plus friends and two pilots, said Fernando Bezerra, a director of the company operating the plane.
The survivors, including a 9-year-old, swam clear of the sinking plane and were helped by local residents until rescuers arrived later Saturday.
One relative, Roberto Buchdid, said his wife called and told him to alert authorities.
"She was very scared," Buchdid said.
The four survivors had only minor injuries and were in good condition Sunday, said Marcelo Alves Cabral, director of the hospital where they were treated. He told Globo's G1 Web site that the worst injury was a deep cut on the back of a 23-year-old man.
"They told me that when they heard one of the engines stop, the plane lost altitude and hit something before going nose-first into the water," Cabral said.
Relatives of the survivors told Brazilian media an engine apparently stopped about an hour into the 185-mile (300-kilometer) flight, which originated in the city of Coari. The plane went down about 50 miles (80 kilometers) short of its destination in Manaus, capital of Amazonas state.
The air force said the pilot contacted air controllers to report driving rain, but he did not mention engine trouble. Brazilian media, citing witnesses and local authorities, said the pilot may have been trying to make an unscheduled landing at the Manacapuru airport less than 1.6 miles (1 kilometer) from the crash site.
Both the pilot and co-pilot died.
It was not clear whether the pilot was trying to make a rare water landing, such as the one last month where a commercial jet went down in New York's Hudson River without a single death among the 155 on board.
Authorities did not know whether everyone on board the Brazilian plane had been accounted for. Rescuers continued to search the area after a witness reported seeing what appeared to be a body being carried off by the current, air force spokesman Lt. Col. Henri Munhoz said.
Munhoz said authorities would investigate whether the plane was carrying more people than allowed.
The air force said the flight plan showed 20 people were on board, but some of the children may not have been listed. The twin turboprop plane has a capacity of 21 people, according to the Web site of Brazil's Civil Aeronautics Agency, or ANAC.
The plane was operated by Manaus Aerotaxi. Bezerra said the aircraft had undergone all of its scheduled maintenance inspections, and ANAC said its documentation was up to date.
Eight divers remained at the scene preparing the plane to be removed from the river. Braga, the firefighter, said it was only partially damaged.
