Eight Corinthians supporters were gunned down at their São Paulo soccer fan club on Sunday 19 April as they were having a barbecue and making flags ahead of a local derby match with rivals Palmeiras.
Three armed men stormed the infamous site known as Pavilhao 9, located beneath a highway overpass, where the Corinthians fan group meets.
According to Brazilian police, the group has close links to Carandiru, a notorious, now-closed prison.
Pavilhao 9 was founded by a group of Corinthians fans who played football with inmates at Brazil's Carandiru prison, known widely for a massacre that left 111 inmates dead in 1992 when military police stormed the facility during a riot.
Shot in the head
Police said witnesses told them that the armed gunmen ordered the victims to lie face down on the ground and then shot seven of them in the head.
An eighth man was shot but tried to escape. He reached a gas station and was later taken to a nearby hospital where he died.
Detective Arlindo Jose Negrao, who is overseeing the raid, ruled out a fight between rival fan groups.
"Through witnesses, we are already exploring a line of investigation, which is not leading us to believe it was caused by fan rivalry," Negrao said at a press conference. "We even have possible suspects."
Negrao added he would not reveal the motive behind the attack, saying that could interfere with the probe.
A club in mourning
The Corinthians team posted the word "Luto" (mourning in Portuguese) against a black backdrop on its official Facebook page.
The former president of the team Andres Sanchez said on the social network he was "deeply saddened to learn of what had happened at Pavilhao 9, with which I had a strong bond since I was a kid."
Much of the soccer violence that breaks out in or near Brazilian stadiums is traced to fan clubs, and Brazilian police often investigates fan groups for ties with drug traffickers.
"It is widely known that criminal organisations are inside of all fan clubs. Organised crime uses these fan groups as a front for their misdeeds," Juca Kfouri, a Brazilian sports commentators, told CBN.
Brazil's Dida and Ronaldo, and the Argentine Carlos Tevez all played for Corinthians.