RIO DE JANEIRO >> Authorities deployed helicopters and other extra security Tuesday in Rio de Janeiro after members of a criminal gang set fire to at least 35 buses in apparent retaliation for the police slaying of one of their key members.

The attack late Monday on public transport buses took place in a western section of Rio far from its tourist districts and caused no casualties, but it represented significant defiance of the government.

Rio’s state Gov. Cláudio Castro said Tuesday after a meeting with his security officials that the city was on “maximum alert” with helicopters and drones reinforcing the police presence on the streets.

Police arrested 12 people in the arson attacks, but six were released due to a lack of evidence as the investigation continues, Castro said.

Authorities said the arson attacks were in retaliation for the police slaying earlier Monday of a man identified as Matheus Silva Resende, the nephew of a criminal leader of the largest militia group in the state.

Militias emerged in the 1990s when they originally were made up mainly of former police officers, firefighters and military men who wanted to combat lawlessness in their neighborhoods. They charged residents for protection and other services, and more recently moved into drug trafficking themselves.

The militias are believed to control about 10% of Rio’s metropolitan area, according to a study last year by non-profit Fogo Cruzado and a security-focused research group at the Fluminense Federal University. These militias are distinct from drug trafficking gangs that control important areas of Rio.