SAO PAULO (REUTERS) - Brazilian police are investigating whether a string of 18 murders in metropolitan Sao Paulo on Thursday night were a coordinated act of revenge by off-duty officers following the nearby deaths of two colleagues in the previous week.

The killings have raised concerns of more reprisals in Brazil, where police are responsible for more than 2,000 deaths per year and off-duty officers rarely face prosecution when they are suspected of vigilante justice.

The same vehicles were identified at the site of multiple shootings in a three-hour period, said Alexandre de Moraes, secretary of public safety for Sao Paulo state, after meeting on Friday with a task force investigating the crimes.

"We haven't ruled out any hypothesis, and the involvement of police officers in the case must be one of the possibilities under consideration," Moraes told journalists, noting the recent deaths of two officers in bungled robberies.

The quick succession of shootings on Thursday came within 10km of each other, in the cities of Osasco and Barueri, western suburbs of Brazil's biggest city and business capital, Sao Paulo.

Security video from one crime scene showed masked shooters arriving in a bar, lining several men up against a wall and opening fire.

Moraes said there was no clear connection between the 18 people killed, six of whom had some kind of criminal record, ranging from domestic abuse to violent assault and drug dealing. Six others were wounded in the shootings.

Initial ballistics evidence suggests the use of at least one 9mm pistol, of the type issued to armed forces in Brazil.