At least 42 inmates have been found dead at four prisons in the capital of Brazil's northern Amazonas state on Monday.
Authorities said the victims appeared to have been killed by "asphyxiation" and were discovered during routine inspections at the prison facilities.
Additional reinforcements have been dispatched to boost security, they added, without providing more details.
This comes a day after 15 prisoners were killed in gang clashes in the same city.

Trump adds seven countries to full travel ban list
Indian parliament votes to allow private firms in nuclear power sector
Doctors in England start five-day walkout during flu surge
Israeli settler kills 16-year-old Palestinian in West Bank, mayor says
Paris' Louvre reopens partially but staff vote to extend strike
Delhi restricts vehicles, office attendance in bid to curb pollution
India summons Bangladesh envoy over security concerns in Dhaka
Alleged Bondi gunman charged with 15 murders as funerals of victims begin
