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 says Iran 'can call', as Iranian FM returns to Pakistan
President Donald Trump said on Sunday that Iran could call if it wanted to negotiate an end to the war launched by the US and Israel, as Iran's foreign minister returned to Pakistan for talks despite the absence of US counterparts.
Ukraine marks 40th anniversary of Chernobyl disaster
Ukraine commemorated the 40th anniversary of the Chernobyl disaster on Sunday, amid fears that Russia's four-year war could spark a repeat of the world's worst nuclear accident that led to thousands of deaths and devastating environmental consequences.
Trump and his officials 'likely targets' of shooting
President Donald Trump and officials in his administration were the likely targets of a suspect who fired on a security agent guarding the White House Correspondents' Association dinner in Washington, US Acting Attorney General Todd Blanche said on Sunday.
Israel issues evacuation warning for seven south Lebanon towns
Israel issued new evacuation orders for southern Lebanon on Sunday after one of its soldiers was killed, warning residents to leave seven towns beyond the "buffer zone" it occupied before a ceasefire that has failed to fully halt hostilities.
Japan deploys 1,400 firefighters to battle raging wildfires in the north
Japan has deployed 1,400 firefighters and 100 Self-Defence Force personnel to battle mountain blazes in the northern part of the country, with the fires, now burning on Sunday for a fifth straight day, continuing to threaten a picturesque coastal town.
His Highness Sheikh Mohammed bin Rashid Al Maktoum, Vice President and Prime Minister of the UAE and Ruler of Dubai, on Sunday announced the establishment of AED1 billion National Industrial Resilience Fund to boost localisation within key industries.
UAE President His Highness Sheikh Mohamed bin Zayed Al Nahyan met on Sunday with Mohamed Ould Cheikh El Ghazouani, President of Mauritania, who is on a working visit to the UAE.
His Highness Sheikh Abdullah bin Zayed Al Nahyan, Deputy Prime Minister and Minister of Foreign Affairs, held a phone call on Sunday with Marco Rubio, US Secretary of State, to discuss the latest regional developments.
