The teenager, whose name was not disclosed, turned himself in to Brazilian police on Monday, a day after he made an emotional confession to a Brazilian television channel with his mother by his side.
His confession came as 12 Corinthians fans remained in custody in Bolivia after being arrested following last Wednesday's match. The South American federation was also expected to rule on the team's appeal against the punishment of having to play its remaining home games in the tournament in an empty stadium.
On Sunday, Corinthians supporters held up a banner honouring the Bolivian boy during a match in the Sao Paulo state championship.
Wearing a jersey of the team's biggest fan group, the teen said in his interview that he was not deliberately aiming the device at the Bolivian fans when it allegedly went off accidentally, hitting Kevin Beltran Espada in the face and killing him.
"When I first pulled the cord to set it off nothing happened, I didn't know how to handle it," he told Globo TV. "When I pulled it again it just went off. I wasn't aiming it, I didn't know it was going to take off like that."
With his face not shown because of his age, the teen apologized and asked for forgiveness.
"When I found out what happened I just thought, 'My life is over, what am I going to do? I just killed a 14-year-old kid,'" he said. "I feel like I'm the worst person in the world. I don't know what I'll do with my life, I deeply regret what happened."
As a minor, the Brazilian cannot be extradited to Bolivia to face charges for the crime. He may just end up facing community service in Brazil.
"A judge will likely give him a social-educational sentence in this first moment," said Ricardo Cabral, the lawyer representing the teen.
The Brazilian was with Cabral and his mother when he turned himself in as dozens of photographers and journalists waited outside the police station. He was expected be released after talking to authorities.
The teen could face up to three years in a juvenile detention centre.
Two of the 12 fans in Bolivia have been formally accused of setting off the flare which killed Espada, while the others are being held as accomplices.
The Brazilian teen said that he didn't know the flare had killed the boy until after the match and decided not to turn himself in to Bolivian police after talking to other members of the group.
"I was scared at the time, I didn't know what to do," he said. "I thought the other guys would be released right away."
He dismissed the notion that he was confessing just to protect those detained in Bolivia, knowing that as a minor he didn't face extradition or a harsher punishment.
"It's just not right for people to pay for what they didn't do," he said. "If I was in their position I also wouldn't want to pay for something I didn't do."
The teen's lawyer said that all the flares seized by Bolivian police from the supporters were bought by his client.
"He left them there in a backpack after the police arrived," Cabral said.
The South American football federation last week said Corinthians, the defending Copa Libertadores champion, will play without its fans for the next 60 days because of what happened in Bolivia.
The team, which had already sold more than 80,000 tickets for its group stage matches, has appealed and another decision by the federation was expected later Monday.
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