Eight Brazilian police officers have been charged with the killing of 30 people in Rio de Janeiro last week.
A total of 12 police officers were being held in connection with the deaths - one of the city's worst massacres.
Investigators believe four officers acted as gunmen while the others played supporting roles.
The 30 men, women and children were killed when gunmen drove through the city's poor suburbs, firing at random.
Last Thursday's attacks began in Nova Iguacu when gunmen opened fire on a crowd at a bar.
Fifteen people were found dead in and around the bar and three more victims died of their injuries in hospital.
The assailants then went on to nearby Queimados where they killed 12 people in two separate shootings.
A .38 calibre gun - the same type used in the shootings - was reportedly found at the house of one of the suspects.
Officials suspect the murders were in revenge for the arrest of eight policemen over another killing.
Human rights groups have compared the shootings to attacks by death squads, which are said to operate in many areas of Brazil.
"It appears to be the result of state authorities cracking down on death squads that have been left for so long uninterrupted that it got to this point," Damian Platt, of Amnesty International, told news agency Associated Press.
Brazilian President Luiz Inacio Lula da Silva has promised to introduce measures to tackle such violence.