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.
Britain pushes allies to boost Ukraine support as Zelenskyy lands in London
Britain on Friday urged fellow allies of Ukraine to reach a deal to use frozen Russian assets to strengthen its hand for any future peace talks, as President Volodymyr Zelenskyy arrived in London for talks with those allies.
EU vows financial backing for Kyiv
European Union leaders agreed on Thursday to meet Ukraine's "pressing financial needs" for the next two years but stopped short of endorsing a plan to use frozen Russian assets to fund a giant loan to Kyiv due to concerns raised by Belgium.
Tourists gawk and recoil at Trump's destruction of the White House East Wing
With many Washington tourist spots closed due to the US government shutdown, some visitors instead are checking out an unexpected and divisive new attraction in the nation's capital: President Donald Trump's surprise demolition of the White House's East Wing.
Japanese police arrest knife man near US embassy days before Trump visit
Police arrested a man who pointed a knife at them in front of the US embassy in Tokyo on Friday, local media reported, as Japan ramped up security ahead of US President Donald Trump's visit in three days' time.
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