The teenager, whose identity was not revealed because he is a minor, allegedly told police he killed his victims over the past three months in a fit or rage or out of revenge, and police inspector Enizaldo Jose Plentz said he believes the boy killed some of his victims for enjoyment.
He said police have enough evidence to prove that the teenager, an unemployed high school dropout, shot dead at least six of the 12 people he says he killed.
Police were still trying to determine if another six people killed in and around Novo Hamburgo were in effect the teenagers' victims.
Investigating the murder of a 39-year-old storeowner, police arrested the boy Wednesday at his home in the city of Novo Hamburgo, Plentz said.
"He told us he shot the storeowner because he had been slapped in the face during an argument the two had a few minutes earlier," Plentz said. "But we think he killed him during a robbery attempt."
"During interrogations, he spoke with the frightening coldbloodedness of someone who enjoys killing people," the inspector said.
The teenage suspect is being held in a juvenile detention center where he will undergo a battery of psychological tests for the next 45 days.
Plentz said police have proof that the boy killed the storeowner and at least five other people with an initial shot to the head, before riddling their bodies with bullets.
"The calmness with which he described the murders and how he would have killed three more if we had not arrested him was scary," Plentz said. "He told us he only killed people who deserved to die like the young man he shot dead because he wanted to date his sister."