At least 27 people have died during a 14-hour riot in a Brazilian prison, police say.
According to sources, the revised death toll was three times what was initially reported - and could increase further.
The violence inside Alcacuz jail, the largest prison in the north-eastern city of Natal, began when members of one of the powerful criminal organisations in most Brazilian jails attacked rivals in a separate wing.
The riot lasted from Saturday afternoon local time until a dawn raid the following day.
Earlier, prison officials said some of the victims were decapitated.
Forensic teams said most victims were decapitated or had body parts chopped off, according to reports. Many bodies were thrown into a ditch.
"We could see the heads ripped off'' three of the dead prisoners, said Zemilton Silva, coordinator of the prison system.
It is not yet known if any prison officers or police staff were injured.
"The operation is still ongoing but the situation is now calm," a spokesperson told BBC Brasil.
Details were still emerging as authorities restored order and removed the dead and injured from the complex.
Footage from the scene showed prisoners roaming the rooftops of the prison, and smoke billowing from at least one building.
It was 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.
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.
During the riot, explosions and gunshots could be heard from outside. Dozens of relatives gathered outside, waiting for information.
Several reports said prisoners cut off electricity to the site at the beginning of the violence.
Rio de Janeiro's O Globo newspaper reported one inmate temporarily escaped, but was quickly recaptured.
No prisoners were thought to be missing, but several were taken to nearby hospitals.
Security forces entered the outer area of the prison, but decided to wait for daylight hours before going in on Sunday morning local time.
Riots are not uncommon in Brazil's overcrowded jails, which are largely controlled by powerful criminal gangs.
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.