The killings have caused alarm among residents

Four of the officers are suspected of direct involvement in last Thursday's massacre, said Rio's top security official Marcelo Itagiba.

Two officers were held on Sunday, but deny any part in the killing.

Officials suspect the seemingly-random murders were revenge for the arrest of eight policemen over another killing.

Some teenagers and a child were among those who died in the shootings in Queimados and Nova Iguacu in Rio's northern outskirts.

Brazilian President Luiz Inacio Lula da Silva has vowed to punish those responsible for the attack.

Two police officers, named as Jose Felipe and Fabiano Lopes, were arrested in their homes on Saturday. Both were said to be on medical leave.

A .38 calibre gun - the same type used in the shootings - was reportedly found at one of the houses.

'Drive-by shooting'

The attacks began in Nova Iguacu, when 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.