Three policemen were detained Sunday after a survivor of Friday's killings of street children identified them as having taken part in the massacre, a police official said Sunday.
Inaldo Santana, deputy chief of the special case squad, said a judge was expected to issue a warrant for the arrest of the three, being held at police headquarters.
The three were picked out of a lineup by Wagner dos Santos, 22, who survived the massacre.
Early Friday, gunmen fired on street children sleeping near Candelaria church in Rio's financial district, killing seven of them.
Dos Santos was critically wounded and only Sunday was he able to leave the hospital briefly to identify the policemen.
The murder of street children is common in Brazil. Human rights organizations estimate there have been an average of two killings a day this year.
Most street children have run away from home because of poverty or family problems. The children, who steal or beg to survive, are seen as nuisances and become targets of death squads.