Eighteen people were killed and seven injured in a wave of overnight shootings in Brazil’s biggest city, Sao Paulo, some of them gunned down in cold blood as they sat in a bar.

Eighteen people were killed and seven injured in a wave of overnight shootings in Brazil's biggest city, Sao Paulo, some of them gunned down in cold blood as they sat in a bar.

Alexandre de Moraes, head of the Sao Paulo state security department, announced the death tally -- originally placed at 19 -- Friday after media reports of a deliberate massacre in the suburbs, possibly by rogue police.

"What I can say for sure is that it's the biggest killing this year," de Moraes said.

Globo News television broadcast security camera footage of one attack, showing a group of masked assailants entering a bar, ordering clients to raise their hands in the air, then shooting them.

Folha de Sao Paulo newspaper quoted witnesses saying that the killers stopped their targets and questioned them about their criminal records, then executed them.

Earlier, a spokesman for the security forces had described the wave of killings late Thursday as "unusual."

The shootings occurred in the Osasco and Barueri areas, reportedly all in a period of about two and a half hours.

Investigators are looking into possible drug-trade connections.