(CNN) -- Rescue operations resumed Thursday morning in Peru after a possibly overloaded ferry capsized in the Amazon River, killing at least 12 of the officially listed 152 people on board, the state-run Andina news agency reported.
Officials believe some people may be trapped in an air pocket inside the overturned ship, Andina said.
At least 171 survivors have been rescued, regional civil defense director Roberto Falcon told CNN en Espaņol.
Two cranes are being used to stabilize the overturned boat, Falcon told the network.
Divers will not start searching for bodies downstream until Friday, the civil defense official said to Andina.
Ten bodies were recovered Wednesday afternoon, before rescue efforts were halted due to darkness, Andina said. Two bodies were recovered Thursday morning. Five remain unidentified, the news agency said.
Overloading may have led to the ship's sinking, Andina said.
A preliminary investigation indicated, Andina said, that the ship had picked up cargo and passengers after leaving port with 146 passengers, six crew and 44 tons of cargo on its manifest.
The captain of the "Camila" motorized vessel stopped twice at fishing villages and picked up animals, fuel containers and a large number of people after setting sail late Tuesday, Andina said. That led to the ship being overloaded beyond its limit, throwing off its center of gravity and stability, Andina said.
It was not immediately known how much extra cargo and how many passengers had been taken on board. The ship had a legal limit of 160 passengers, Andina said.
A 25-year-old Italian passenger who survived described vast overcrowding.
"It was incredible the way we left the fishing village in a voyage in which you couldn't even get out to go to the bathroom," Andina quoted Ricardo Crispo as saying. "Hammock over hammock and one person on top of another. Of course, we all wanted to get to the end of the trip."
The ship started to lose control, Crispo said, noting that he and two friends stationed themselves near the ship's rail.
"That's why we are alive," he said.
The Peruvian Coast Guard was alerted at 2:40 a.m. Wednesday that the ship was sinking due to overloading, Andina said.
"It was raining almost all night," Crispo said. "When we arrived at this port of Santa Rosa, the ship began to list to one side and all of a sudden it sounded like it was sucking in water. All this happened at the stern, by the engine, and that's how the 'Camila' overturned."