Brazilian President Luiz Inacio Lula da Silva has condemned as "barbaric" the killing of 30 people by gunmen in the northern outskirts of Rio de Janeiro.
The president, known as Lula, said the government would spare no effort to find and punish those responsible.
Several teenagers and a child were among those gunned down on Thursday.
Rio de Janeiro state security secretary Marcelo Itagiba said it appeared the gunmen were police, and that the shootings could have been a reprisal.
Eight police officers were arrested on suspicion of murder on Thursday before the seemingly random shootings in Queimados and Nova Iguacu.
However, the Brazilian federal Justice Minister, Thomas Bastos, said it was too early to be sure, and all lines of investigation had to be followed.
"The government will spare no effort to find and punish those responsible for this barbaric, cowardly crime," Lula said.
Police have released composite sketches of two suspects, reports say.
'Drive-by shooting'
Grieving families have begun burying the victims of Thursday's massacre, including several teenagers and a child, amid calls for justice.
In Nova Iguacu, the gunmen fired on a crowd at a street-corner bar, in what Brazilian media describe as a drive-by shooting.
Fifteen people were found dead in and around the bar and three more victims died of their injuries in the hospital on Friday.
The gunmen, possibly joined by a second car, then drove to the Queimados neighbourhood where they killed 12 people in two separate shootings.
Rio is one of the most violent cities in the world.
Rival drugs gangs are in control of many slum areas, known as favelas.
Rio police have long been accused by human rights groups of using extreme brutality and acting with impunity.
They say police carried out a 1993 massacre in the state, in which 21 people were killed.
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