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.
Kuwaiti forces deal with missile, drone attacks on Monday
Kuwait's Chief of the General Army Staff announced its air defence forces intercepted hostile missile and drone attacks early on Monday.
US says it struck Iranian drone command sites at the weekend
The US has stated on Sunday it conducted "self-defense strikes" on Iranian radar and drone control sites in Iran's Goruk and Qeshm Island at the weekend in what it said was a response to "aggressive" actions from Tehran.
US proposes new plan to ease Israel-Lebanon tensions amid fighting
US Secretary of State Marco Rubio has spoke with both Lebanese President Joseph Aoun and Israeli Prime Minister Benjamin Netanyahu on the diplomatic negotiations between Israel and Lebanon and has proposed a plan to allow for "gradual de-escalation", a US official said on Sunday.
At least two dead at Hanwha Aerospace fire in South Korea, official says
At least two people have died and two others were injured in a fire at a factory operated by South Korea's Hanwha Aerospace in the city of Daejeon, a fire official said on Monday.
Netanyahu orders deeper Israeli incursion into Lebanon
Israeli Prime Minister Benjamin Netanyahu said on Sunday he had ordered troops to move further into Lebanon in the battle against Hezbollah, despite a ceasefire announced more than six weeks ago.
