Investigators in Brazil are probing whether police officers gunned down at least 12 young men in separate incidents within hours of one another on the outskirts of a university town to avenge the murder of a colleague.
The wave of killings began on Sunday night and continued into Monday in Campinas, a city of one million people known as the hub of Brazil's tech industry.
The mostly drive-by shootings of the young men occurred within about four hours of one another and came after an off-duty police officer was killed while fighting armed robbers who targeted him as he stopped at a petrol station in the same region.
"We're not ruling out executions, revenge nor a fight between criminals," police investigator Licurgo Nunes Costa told the newspaper Estado de S Paulo.
"It's a sequence of events in the same region and around the same time and we have to consider the relationship between the occurrences."
Police in Campinas, about 60 miles north west of Brazil's biggest city, Sao Paulo, said the killings took place, but would not provide details.
About 30 people protested over the killings, attacking commuter buses and burning at least three of them. There were no reported injuries.