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William M. Raines High School was hosting Robert E. Lee High School.
Joerod James Adams, 19, was killed, but the two other victims, a 16-year-old female student and a 17-year-old male student, are expected to survive their injuries, police said.
Investigative efforts suspect the male victims were at the game together and were the direct targets of the shooting.
The female victim appeared to have been struck by a stray round, according to police.
The JSO said the male victims have ties to known criminal street gang members.
The Sheriff's Office said there were several fights and disturbances during the football game, and they said they will be looking into those to see if there are any connections to the shooting.
The JSO estimated that 4,000 people attended the season opener and is asking again for witnesses to come forward.
"With the amount of people there, we should have many witnesses," the JSO said.
Jacksonville Mayor Lenny Curry released the following statement to Action News Jax:
"Last night, a Friday night gathering of sports and community fellowship at a high school was devastated by senseless gang violence. In the hours since, I have discussed the incident with Sheriff Williams, Duval County Schools Superintendent Dr. Greene, as well as other city officials and faith leaders. I have stressed that we must redouble our commitment to showing young people that crime and violence is not the path. And for those who do not understand that fact, I will continue my support for law enforcement personnel and programs that ensure that perpetrators of crime face the full force of the criminal justice system. As mayor, I will continue this fight to rid our streets of violent gangs."
Duval County Public Schools said additional security and specialty grief counselors will be at Raines and Lee during the school week.