At least 26 people have died during a 14-hour riot in a Brazilian prison, police said.
The revised death toll is three times what was initially reported, and could increase further, the sources said.
On Saturday afternoon, violence erupted between members of rival gangs at Alcacuz prison, starting a riot which lasted until a dawn raid the following day.
Earlier, prison officials said some of the victims had been decapitated.
‘We could see the heads ripped off’' three of the dead prisoners, said Zemilton Silva, coordinator of the prison system.
Forensic teams said most victims had been decapitated or had body parts chopped off, according to reports, and many bodies were thrown into a ditch.
It is not yet known if any prison officers or police staff were injured during the incident.
The riot is believed to have started when members of one of Brazil's powerful criminal organisations attacked their rivals, who were housed in a separate wing.
Details are still emerging as authorities restore order and remove the dead and injured from the complex.
Video footage from the scene showed prisoners roaming the rooftops of the prison, and smoke billowing from at least one building.
Several reports said prisoners had cut off electricity to the site at the beginning of the violence,
Rio de Janeiro's O Globo newspaper reported one inmate had temporarily escaped, but was quickly recaptured.
No prisoners are believed to be missing, but several were taken to nearby hospitals.
Alcacuz prison is designed to hold 620 inmates, but now holds more than 1,000 in overcrowded conditions - a common problem throughout the country's prison system.
This is the third major riot in Brazil this year. Nearly 100 inmates died in riots earlier this month in prisons in the states of Amazonas and Roraima.
Riots are not uncommon in Brazil's overcrowded jails, which are largely controlled by powerful criminal gangs.
But the scale of the violence this year has put pressure on President Michel Temer to address the problem.
After the Amazonas prison riot, the government announced plans to build five more high security jails and to create new intelligence units to try to curb the power of the gangs behind bars.
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