RIO DE JANEIRO (AP) — Forensic experts in the Brazilian city of Altamira where 58 inmates were killed in a bloody prison riot said they had identified 17 bodies so far, with relatives facing the grim task of identifying sometimes decapitated corpses.

Only 15 of the bodies had been released to family members by late Tuesday. Workers at the coroner's office said they were slowed by the size of the facility as well as problems with lighting that meant they had to stop working at 6:30 p.m. In the Amazon heat, the bodies were being kept in a large refrigerated truck.

As the slow process of identifying the bodies from Monday's clash between rival gangs members at Altamira prison advanced, dozens of frustrated family members spent the day waiting outside the city's forensic institute.

There, forensic expert Marcel Ferreira described the anxiety among the waiting relatives and said some passed out when called on to identify the bodies of beheaded loved ones. Sixteen of the dead inmates had been decapitated.

In a bid to prevent further violence at Altamira in Para state, authorities began transferring all 46 inmates accused of being involved in the violence to other prisons, including stricter federal ones. Local authorities confirmed by late Tuesday at least 33 inmates had been moved to the state capital of Belem, from where they would be sent to other jails.

State officials said clashes erupted in Altamira early Monday when the local Comando Classe A gang attacked a wing of the prison where members of rival gang Comando Vermelho, or Red Command, were held.

In many of Brazil's prisons, badly outnumbered guards struggle to retain power over an ever-growing population of inmates, with jailed gang leaders often able to run their criminal activities from behind bars.

Comando Classe A members allegedly set fire to the temporary containers where inmates belonging to Red Command were being held while construction of another wing was underway. Most of the victims died of asphyxiation.

"This is clearly a declaration of war on the Red Command," said Jean-François Deluchey, adjunct professor in political science at the Federal University of Para who has been studying the region for 20 years.

Authorities have not yet revealed the exact cause of the latest violence in Altamira, only confirming that it was a fight between criminal groups. But several recent prison massacres have been attributed to gangs clashing over control of drug-trafficking routes in the multibillion-dollar Amazon drug trade.

In May, two days of unrest in the neighboring state of Amazonas left 55 prisoners dead in four different prisons of that state's capital, Manaus. In 2017, more than 120 inmates died in prisons across several northern states.

"It's the same logic, the same movement," Deluchey said. According to him, Red Command has a strong presence in the north and is trying to expand further in the region.

Deluchey says it is hard to confirm with certainty but initial reports indicated that Comando Classe A, a local gang thought to have been created not long ago inside the Altamira prison, is linked to another powerful Brazilian gang, First Capital Command.

"The First Capital Command is losing grounds and it looks like Comando Classe A is helping them stop the hegemony of Red Command," he said.

The professor said he had already seen promises of retaliations by members of Red Command for Monday's attack.

Gruesome violence is often used in Brazilian prisons to gain respect and send a strong message to new arrivals, he said. "Violence is to impress, to frighten, so that new (inmates) join the side of those who decapitate, and not the decapitated."

The latest killings represent a challenge for the far-right administration of President Jair Bolsonaro. A former army captain, Bolsonaro ran a tough-on-crime campaign, promising to curb epidemic violence in Brazil, including in its overcrowded and out-of-control prisons.

The president publicly addressed the killings Tuesday in a video published on the online news portal G1. Asked by journalists whether security should be strengthened at Altamira prison, Bolsonaro replied: "Ask the victims of those who died in there what they think."

Brazil has the world's third-largest prison population, behind the United States and China, with more than 720,000 individuals behind bars, according to official data from 2017. Some Brazilian prisons have more than three times as many inmates as their maximum capacity.

At Altamira, a local judge revealed in a July report examined by The Associated Press that he had counted 343 detainees in a facility authorized for a maximum of 163 people.

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Associated Press photojournalist Raimundo Pacco in Altamira contributed to this report.