Sao Paulo , Brazil ( CNN ) -- A five-hour killing rampage in Brazil 's southeastern city of Campinas that left 12 people dead is being investigated amid claims it may have been sparked by the earlier death of a police officer . The 12 victims were gunned down in several separate incidents on the outskirts of the city , in what Brazilian media said appeared to be execution-style killings . Authorities say they are looking into all possibilities , including whether the killings were gang-related . But they also say the timing and location of the shootings add weight to a theory that they were revenge killings carried out by police . The slayings occurred between Sunday night and Monday morning , just hours after an off-duty military police officer , Arides Luiz dos Santos , was killed fighting off an attempted robbery at a gas station . According to Brazilian media , in one of the cases , a car pulled up , its occupants told children to go inside and then opened fire . Hundreds of angry residents took to the streets to protest the killings . They attacked buses , burning three of them . Police have stepped up their presence in Campinas , located about 50 miles northwest of Sao Paulo , to prevent further violence . The latest deadly incidents come as Brazil prepares to host the 2014 World Cup in June . None of the venues for the soccer tournament is in Campinas . However , fans from around the world are expected to flock to nearby Sao Paulo , the country 's financial and business hub . Hooded men . Fernando Grella Vieira , the secretary of public safety for Sao Paulo , said Tuesday that the civil and military police `` will not rest '' until they have found those responsible for all 13 deaths in Campinas . No possibility is being ruled out during the investigation , he said on a visit to the city . `` These are intolerable crimes and I can guarantee that the police will not rest until we solve these deaths and arrest the perpetrators , '' he added . Grella said it was too early to comment on suspicions raised by witnesses of the alleged involvement of military officers in 12 of the killings . But he stressed that he does not tolerate abuse or crimes committed by state agents . Nine of the victims died in two separate incidents , according to the state-run Agencia Brasil . Hooded men reportedly used pistols to shoot the victims , aged between 17 and 30 , in the head . The remaining three were killed in separate attacks , the news agency said . At least six of the victims had a criminal background , with offenses including drug trafficking , murder and auto theft , the news agency cited Licurgo Costa , director of the Judiciary Police department , as saying . Victims 'hit in the head , chest or abdomen ' The chief of the Civil Police 's homicide division , Devanir Dutra , told CNN affiliate TV Globo that the police will take accounts from at least 20 witnesses beginning Wednesday . `` Let 's start with the relatives and friends of the victims , '' he said . According to Dutra , two people were injured and remain hospitalized in Campinas . They may be summoned by the police as witnesses . Dutra said that the number of shots fired signaled executions . `` They ( the victims ) were hit in the head , chest or abdomen , '' he said . One hypothesis being examined by the Civil Police is rivalry between drug gangs , he said . CNN 's Shasta Darlington reported from Sao Paulo and Marilia Brocchetto from Atlanta , while Laura Smith-Spark wrote in London .
Police `` will not rest '' until they find those behind killings in Campinas , official says . Authorities are looking into all possibilities , including whether it was a police revenge attack . 12 people were shot dead , hours after an off-duty officer was killed in an attempted robbery . Angry residents took to the streets and set buses on fire to protest the killings .