Forty inmates have been found dead at four separate prisons in Manaus, the capital of Amazonas state in northern Brazil.
Prison officials said all the bodies, which were found during routine inspections, showed signs of asphyxiation.
Monday’s violence came a day after 15 prisoners were killed in jail clashes in Manaus.
The state governor said a task force had been sent to Amazonas to help control the disturbances.
Prison officials said that the victims belonged to the same drug trafficking group and that they had been killed due to an internal gang rift.
Robert Muggah, who is research director at the Igarape Institute think tank in Rio de Janeiro, told newsmen that he thought a “settling of scores” was behind the killings.
Fifteen inmates died in clashes in Anísio Jobim jail when prisoners turned on each other. Some were stabbed with sharpened toothbrushes while others were strangled.
The violence broke out during visiting hours and the mother of one inmate described it as “total chaos”.
“Everyone started to run, and everyone was pounding on the cell gates, at the doors, and running down the corridors.”
Col Marcus Vinicius, who is in charge of prisons in Amazonas state, said “It was not a rebellion, it was a fight among inmates’’
He said the fact that the killings had happened during visiting hours broke an unwritten prison rule of “never to kill during a family visit”.
Col Vinicius said murders inside jails were inevitable: “We have to have the maturity to understand that in any prison in the world, when someone wants to kill, they will kill.”