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.
US military says it turned away blockade runner trying to reach Iranian port
The US military has announced that it fired a missile into the engine room of a vessel trying to get through the US blockade of Iran on Friday.
Illegal mine collapses in China, killing five just days after Shanxi disaster
A mine shaft collapse during an illegal mining operation in China's southwestern Yunnan province killed five people and injured one, state media reported, days after the country's deadliest mining accident since 2009 left at least 82 dead.
Israel troops capture Beaufort Castle in southern Lebanon push against Hezbollah
Israeli troops have seized the 900-year-old Beaufort Castle and its strategic ridge in southern Lebanon, the military said on Sunday, a major advance against Hezbollah despite a ceasefire announced more than six weeks ago.
Jubilant PSG supporters spill onto Paris streets, some clash with police
Paris police deployed thousands of officers to control crowds at some of the city's hot spots, using teargas and arresting dozens following the win of the city's football club Paris St Germain in Saturday's Champions League final.
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