Brazil police investigated over three-hour killing spree in Campinas

An investigation is under way in Brazil to find out whether a group of policemen went on a three-hour killing spree in an alleged revenge attack.

Twelve people were killed overnight by gunmen in vehicles in a district of Campinas, a city north of Sao Paulo.

The attacks began hours after an off-duty policeman died in a robbery.

Investigators told the BBC that a revenge attack by fellow military policemen is regarded as "the strongest theory".

In one incident, the attackers stopped their car next to a group of people, asked children to be taken away and fired on the others, killing several people.

The calibre of weapons used in the attacks is the same as that used by military police, a source told the BBC.

Extra police were deployed to the neighbourhood where the incidents took place

Several of those who died had criminal records, local media reported.

Investigators believe it is significant that the attacks "happened in a sequence, in the same area of the city and in the space of a few hours," local police commander told Licurgo Nunes Costa O Estado de Sao Paulo newspaper.

He said other possibilities are being considered, including "an extrajudicial execution or even a conflict between rival criminal gangs".

The shootings sparked an attack at the Campinas bus terminal, located in the area where the killings took place, said the BBC's Gary Duffy.

Around lunch time, a group of more than 20 masked men set fire to three buses and one car at the terminal. Seven other vehicles were vandalised.

Campinas, with a population of more than 1 million people, is located 100 km (60 miles) from Sao Paulo - Brazil's most populous city.