(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.

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