The shooting occurred during the Madden 19 Tournament at the GLHF Game Bar, according to the Twitter of CompLexity Gaming, one of the event organizers.
Preliminary news reports say 11 people have been shot and witnesses say there are four dead.
"There were three deceased individuals at the scene, one of those being the suspect, who took his own life", Sheriff Mike Williams told a news conference. When the loud bangs kept coming, the Chicago couple and fellow video gamers attending a weekend tournament recognized them as gunfire and began scrambling for an exit.
EA Sports, a video game developer and publisher, condemned the shooting as "a senseless act of violence".
SWAT teams conducted a methodical search of the Landing and the immediate area but found no additional gunmen and evacuated the mall of all but law enforcement personnel by 4 p.m. "Please don't come running out".
Police added that the public should stay away from the area and that there were "multiple fatalities at the scene".
"The latest line we're getting from the Jacksonville police is the suspect in this shooting is now dead", Al Jazeera's Andy Gallagher reported from Miami.
The sheriff's office used Twitter and Facebook immediately after the shooting to warn people to stay far away and to ask anyone who was hiding to call 911.
Florida Gov. Rick Scott tweeted that he had been in touch with Jacksonville Mayor Lenny Curry to offer "state support and resources" in response to the shooting. Multiple shots are then heard, followed by screams.
"Whatever we're doing, we're streaming it", explained local gamer Matthew Gilliam. What I can say is I know today what lifelong Jacksonville residents have known forever - we are resilient, compassionate and remarkably supportive of our neighbors and friends, particularly in the most trying occasions. "Love you all and hope something like this never happens again". A second person with gunshot wounds followed shortly behind them.
Electronic Arts, the company that makes the Madden 19 game, tweeted a statement in the hours following the shooting, sending sympathies to the victims' families. The Landing is an open-air marketplace with stores, bars and restaurants in downtown Jacksonville along the St. Johns River.