At least 19 people were killed and seven injured in a series of shootings in Brazil's Sao Paulo, official sources revealed late Friday.Alexandre de Moraes, secretary of public security in the state of Sao Paulo, said the shootings took place late Thursday and were the "largest killing of the year". A total of 15 were killed in the city of Osasco, three in Barueri and one in Itapevi, all in the metropolitan area of South America's largest city, Xinhua news agency reported. A 9 mm pistol, commonly used by the Brazilian army, and 38 and 380 mm guns, as used by the police, were used in the attacks at 12 different sites, feeding suspicions of police and military involvement. According to de Moraes, investigators are working from the hypothesis that the killing spree could have been carried out by corrupt cops. They could have been seeking to avenge the murders of a policeman and a civil guard officer in Osasco and Barueri in recent days.Witness statements show that all the attacks were carried out by masked men in a car. There was also report that the car was a silver Peugeot.