Dozens of people were attending a Madden 19 video game tournament event at the Jacksonville Landing when someone started shooting around 1:45 p.m. Sunday.
The Madden Championship Series competition was going on at the Good Luck Have Fun Game Bar, which is next to the Chicago Pizza at the popular riverfront destination.
"This is a horrible situation, and our deepest sympathies go out to all involved," event organizers said.
At least one suspect is dead, the Jacksonville Sheriff's Office said. Sources said four people were killed and at least 11 injured.
Video taken from the scene captured the sound of multiple gunshots as two players were engaged in competition.
The two-day event is a series of competitions held in major cities.