The Para state public security office said the four were discovered dead when the prison vehicle arrived in the town of Maraba. They said vehicle had four compartments and was carrying 30 handcuffed inmates who were suspected of involvement in Monday's clash gangs at the Altamira prison.
Authorities said the four who died were from the same gang and said they are investigating. The prisoners were among 46 being sent to other prisons, including stricter federal ones. Local authorities confirmed late Tuesday at least 33 inmates had been moved to the state capital of Belem, from where they would be sent to other jails.
Only 15 of the bodies from the riot had been released to family members by late Tuesday. Workers at the coroner's office said they were slowed by the small 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. Dozens of frustrated family members spent the day waiting outside the morgue, and forensic expert Marcel Ferreira said some passed out when called on to identify the bodies of beheaded loved ones. Sixteen of the inmates had been decapitated.
State officials said clashes erupted in Altamira early Monday when the local Comando Classe A gang attacked a wing of the prison holding members of the rival Comando Vermelho, or Red Command. In many of Brazil's prisons, badly outnumbered guards struggle to retain control 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 motive for the clash, only confirming that it was a fight between criminal groups. But several recent prison massacres have been attributed to gangs battling to control 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 said it is hard to confirm with certainty, but initial reports indicated that Comando Classe A, a local gang thought to have been created recently 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 killings represent a challenge for the far-right administration of President Jair Bolsonaro. 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 G1 news portal. 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.
Associated Press photojournalist Raimundo Pacco in Altamira contributed to this report.