(CNN)Fifty-five inmates have died in gang-related riots at four prisons in western Brazil, according to authorities.
Fifteen prisoners were found dead Sunday and 40 more Monday in the city of Manaus, the capital of the state of Amazonas, the region's Governor Wilson Lima said.
Lima said authorities believe the deaths are a result of violent clashes among rival factions within the same drug gang, known as the Family of the North. Nine gang leaders are being transferred to maximum security prisons.
A prison official said on local radio that some of the inmates killed at the Anisio Jobim penitentiary on Sunday were choked, while others were stabbed with sharpened toothbrushes.
Meanwhile, the prisoners who died on Monday were killed by asphyxiation, according to Lima, who added that they were strangled by their cellmates. Folha de S. Paulo newspaper reported that sheets were used to strangle the prisoners.
The total death toll has been revised down from 57, as previously reported by Agencia Brasil news agency citing Amazonas officials.
Human rights groups have accused the government of doing too little to prevent the violence at prisons that have become recruitment centers for gangs -- and even facilitating clashes by allowing the cells to become overcrowded.
According to Conectas human rights group, the Anisio Jobim penitentiary has capacity for 454 prisoners but was housing more than double that when the violence started on Sunday.
The clashes pose a challenge to the new far-right President Jair Bolsonaro, but could also end up giving his approval rating a boost as he is seen as tough on crime and has previously vowed to crack down on criminal gangs and prison violence.
Brazil's justice ministry is sending in a task force to take over control of the prisons in Amazonas. Lima said that 200 inmates who could potentially be targeted have been separated and are under protection.
"I just spoke to Justice Minister Sergio Moro and he is already sending a prison intervention team to the state of Amazonas to help us in the critical moment with a problem that is national: the problem with the prisons," Lima said.
Brazil was rocked by a spate of prison riots in early 2017 in which almost 100 inmates died. Those riots prompted calls for the federal government to tackle what analysts described as systemic failures in the country's prison system.