At least 42 prisoners have been found dead at four prisons in the northern Amazon jungle city of Manaus on Monday, authorities said.

The deaths came one day after 15 inmates were killed in clashes between rival prison gangs in the same city.

The Amazonas state government said in a statement that the victims appeared to have been killed by “asphyxiation.” It added that federal authorities had dispatched reinforcements to the state to boost security.

On Sunday, 15 inmates were killed when fighting broke out at the Anisio Jobim Prison Complex in Manaus. Some of the 42 prisoners killed on Monday were also discovered at the facility, while the rest were found at three other jails.

Federal authorities dispatched security forces to the Anisio Jobim Prison Complex

Overcrowding, drug violence

Severe overcrowding is a huge problem in Brazil’s prison system, which is plagued by drug violence and riots. In 2017, more than 120 prisoners died during three weeks of clashes between rival prison factions in northern Brazil.

Brazil has one of the world’s largest prison populations. According to official statistics, there were more than 700,000 inmates as of 2016 — double the capacity of the nation’s jails.

“Overcrowding and understaffing make it impossible for prison authorities to maintain control within many prisons, leaving detainees vulnerable to violence and recruitment into gangs,” Human Rights Watch said in a recent report.