About 60 inmates were killed in a prison riot in northern Brazil that ended early Monday, one of the bloodiest disasters ever in the country’s crowded penitentiary system, officials said.

The prison brawl began Sunday in the Amazonian city of Manaus and flared into a gruesome nightlong battle, with several inmates decapitated and the bodies of others burned, officials said.

The riot was sparked by a clash between two gangs in the prison and reflects shifting allegiances among Brazil’s powerful narco-trafficking groups, authorities said.

Sergio Fontes, the head of security in Amazonas state, said that a local gang, aided by former inmates out on parole, smuggled guns into the prison and took 74 prisoners and 12 guards hostage.

“This was one more chapter in the silent and imperious narco-trafficking war,” Fontes said.

Officials gave varying figures for the death toll from the disturbance at the Anisio Jobim penitentiary, with state officials at one point saying at least 60 inmates were killed. By Monday evening, Brazilian media, citing local police, put the number of dead between 50 and 80. The Associated Press meanwhile said the state public security secretary’s office had revised its earlier death toll to 56.

At least six decapitated bodies were discovered and over 100 prisoners escaped in the aftermath of the riot, according to the local chapter of the Brazilian bar association.

“We witnessed the bloodiest night in the history of our state penitentiary system,” the association’s president, Marco Aurelio Choy, told the Brazilian news site Globo.

The jail massacre appeared to be the second-deadliest in Brazil’s history, after a 1992 episode in which more than 100 inmates at a prison in Sao Paulo were fatally shot, many by police trying to put down a revolt.

After the violence was quelled Monday, bodies were piled in the prison corridors and the hallways were slick with blood, State Penitentiary Minister Pedro Florêncio told Globo. Throughout the night, inmates’ distraught family members milled outside the prison, waiting for news of their loved ones.

Human rights groups have assailed the conditions in Brazil’s prisons, many of which are crowded and known for violence. Human Rights Watch reported last year that the number of adult prisoners had grown 80 percent in a decade, to more than 600,000, while staffing levels had not kept pace. That has allowed gang activity to flourish, it said.

At the time of this weekend’s riot, the Anisio Jobim prison housed three times as many inmates as it was designed to hold, officials said. A state inspection of the prison conducted in October classified it as “terrible.” Prisoners had no access to legal aid, health care or education, according to the report.

Robert Muggah, research director of the Igarapé Institute, a Rio de Janeiro think tank, said the deadly clash reflected the changing politics among Brazil’s drug factions.

“Prisons, which have always been tinderboxes, are manifesting the reorganization of organized crime in Brazil,” he said.

Officials said the riot started when inmates associated with a local gang called the Northern Family attacked prisoners with ties to Brazil’s largest narco-trafficking gang, the Sao Paulo-based First Capital Command.

In October, the First Capital Command gang ended an alliance with the country’s second-most-powerful criminal group, Red Command, which is based in Rio de Janeiro, according to the website InSight Crime, which monitors violence in Latin America.

Since then, fights in several Brazilian prisons have been blamed on the new rivalry between the groups or their allies. The Manaus-based gang involved in the latest prison violence is an ally of Red Command, according to Reuters.

In mid-October, riots erupted in two Brazilian prisons, leaving at least 18 people dead. Two groups of inmates clashed in one facility in the northern state of Roraima on Oct. 16, leaving 10 dead, several of them decapitated, according to news reports. A day later, eight inmates died in fighting at a prison in the northwestern state of Rondonia, according to news reports.

Last week, President Michel Temer announced he would secure $360 million to improve prison infrastructure and security. Amazonas’s governor has pledged to use part of that money to improve Anisio Jobim.

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