(CNN)Fifty-seven inmates have died in riots at four prisons in western Brazil, authorities say.

Officials told the Agencia Brasil news agency that 15 prisoners were found dead Sunday and another 42 Monday in the city of Manaus, the capital of the state of Amazonas. Many appeared to have been asphyxiated. Police did not say what prompted the riots.The Ministry of Justice and Public Safety said it would send a task force to the region, while the state prison secretary also announced an investigation. 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 Brazilian prison system.

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