"Three deceased individuals were located at the scene, one being the suspect", Williams said, adding that 11 people suffered gunshot wounds while two more were injured while fleeing the scene. "STAY AWAY #TheLandingMassShooting", the Jacksonville Sheriff's Office said on Twitter.
In disturbing footage apparently captured as part of a live stream on the video game streaming website Twitch, several gunshots could be heard in the background, before the stream disconnected. He is believed to have stayed in area the previous evening, according to investigators. "The area is not safe at this time", the office said.
A further tweet said: "We are finding many people hiding in locked areas at The Landing".
Jacksonville Sheriff Mike Williams named the shooter as David Katz, 24, of Baltimore and said he was in town for the competition.
A suspect involved in the shooting is dead.
EA Sports has used the NFL licence in its Madden football game series for more than 25 years. Jason Lake, the founder and CEO of compLexity said on Twitter that 19-year-old Drini Gjoka, was shot in the thumb.
Seven had been shot, Williams said.
A competitor at Sunday's tournament told the LA Times on Sunday that the shooter was a participant who had been disqualified earlier in the day, but authorities have not confirmed the testimony.
"The tourney just got shot up", Gjoka tweeted shortly after 1:30 p.m.
"Jacksonville is mourning", he said.
White House spokeswoman Sarah Huckabee Sanders said President Donald Trump had been briefed on the attack and the White House was monitoring the situation.