SAO PAULO (AFP) - A Brazilian police commander said on Tuesday that there was strong evidence that fellow officers may have been involved in a spate of killings in the southeastern Brazilian city of Campinas.

On Monday, ten public buses were destroyed in Campinas in rioting that erupted after 12 people were shot and killed in murky circumstances.

Mr Julio Cesar Fernandes Neves, the recently arrived police inspector general of Sao Paulo state, said nine out of ten people who have talked investigators said they suspected police involvement.

"Unfortunately I am assuming my new post as inspector general with this case," he added.

The surge of violence began with the murder of an off-duty policeman, according to police and press reports.

A spokesman said on Monday that police were investigating various possible motives for the shootings "including revenge, conflicts between rival gangs or even police executions".

A group of about 20 assailants, most them hooded and armed with sticks and stones, ransacked a Campinas bus terminal where five of the overnight deaths occurred.

They set fire to three buses and ransacked seven others, according to police.

The daily O Estado de Sao Paulo, reported on Monday that the killings took place hours after an off-duty policeman was killed during a robbery attempt at a gas station in the area.

No arrests have been reported so far.

Campinas is 90km from Sao Paulo, which will host the opening game of the World Cup on Jun 12.

In late 2012, the state of Sao Paulo was rocked by a wave of violence that left scores of people dead, including several police officers.

Experts attributed the mayhem to an undeclared war between a prison-based criminal gang known as the First Command of the Capital and police.

Last July, a survey on violence published by the Latin American Studies Center said one million people were killed in Brazil between 1980 and 2011, making it the world's seventh most violent nation.