In live footage of the tournament, players can be seen reacting to the gunfire and heard crying out before the footage cuts off.
Police said one suspect was dead but it was “unknown” if there was any one else involved in the shooting.
Gamer Drini Gjoka tweeted his relief to be alive after police urged people to stay away from the area after “multiple fatalities” at The Jacksonville Landing entertainment complex.
He said: “The tourney just got shot up. I’m leaving and never coming back. I am literally so lucky. The bullet hit my thumb.”
Yesterday Jacksonville police tweeted: “Searches are being conducted. We are finding many people hiding in locked areas at The Landing. We ask you to stay calm, stay where you are hiding. SWAT is doing a methodical search inside The Landing.
“We will get to you. Please don’t come running out.”
The first game in the Madden NFL franchise was released 30 years ago named after legendary coach and commentator John Madden.
Gaming company EA Sports tweeted: “We are working with authorities to gather facts at this stage. This is a horrible situation, and our deepest sympathies go out to all involved.”