Twelve people have been killed in the Brazilian city of Campinas in a surge of violence that began with the murder of an off-duty policeman.
Police say they are investigating various possible motives, "including revenge, conflicts between rival gangs or even police executions".
A bus terminal in the southeastern Brazilian city emerged as a focus of the violence with police reporting that five of the deaths overnight occurred there.
Some of the killings were perpetrated by armed criminals riding in vehicles, police said.
"We have not established whether these deaths are related. All we know for now is that they occurred in the same area of Campinas," Sao Paulo state's third largest city, the spokesman noted.
The news portal G1, quoting local police sources, said three of the victims were rushed to hospital but died of their wounds while the other died on the scene.
The daily O Estado de Sao Paulo, meanwhile, said the killings occurred hours after an off-duty policeman was killed during a robbery attempt at a petrol station in the area.
No arrests have been reported so far.
The Vida Nova bus terminal again saw violence at around midday on Monday, when it came under attack from a group of 10 to 20 men who set fire to three buses, prompting police to intervene.
Campinas is located 90km from Sao Paulo, the state capital which will host the opening game of the World Cup on June 12.
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.