The Ceara supporters were shot in the head after a confrontation with Fortaleza fans about five kilometers from the stadium, with witnesses telling police the victims were on their way to the stadium when rival fans on a motorcycle opened fire.
"We lament what happened. But there is work being done by the police and the army in many areas of security, so we are not concerned with that for the Confederations Cup," World Cup organizing committee member Tiago Paes was quoted as saying by AP.
Authorities later detained a man on a motorcycle near the crime scene who allegedly carried a gun, but no details about the suspect's identity have been released as of yet.
Furthermore, over 180 people were arrested for vandalism and disorderly conduct before Sunday's match.
The Castelao is one of the six stadiums hosting games for this summer's tournament, and one of 12 venues for next year's World Cup.