The shocking video shows gameplay from a Madden 19 tournament with inset footage of the two gamers going head-to-head.
A few seconds into the video the sound of shooting begins. At least 11 shots are fired.
The inset image of the gamers, who have not been identified, disappears from screen before a notice appears alerting viewers that the players have stopped playing.
After the first few shots the sound of a struggle can be heard.
Shouts and swearing can also be heard between gunshots on the disturbing video.
The local sheriff’s department has confirmed there are multiple fatalities at the scene.
Jacksonville Sheriff’s Office said on Twitter: “Mass shooting at the Jacksonville Landing. Stay far away from the area. The area is not safe at this time. STAY AWAY
“Multiple fatalities at the scene, many transported.
“One suspect is dead at the scene, unknown at this time if we have a second suspect. Searches are being conducted.”
A spokesman for Jacksonville’s Memorial Hospital, Peter Moberg, said it was treating three victims, all of whom were in stable condition.
Florida Governor Rick Scott, a Republican who is challenging longtime Democratic Senator Bill Nelson in November’s election, said he had offered to provide local authorities with any state resources they might need.
Florida Senator Marco Rubio said both the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were coordinating with local authorities to provide assistance.
President Donald Trump has been briefed and is monitoring the situation in Jacksonville, the White House said.
Reacting to news of the shooting during the tournament involving its video game, Madden 19 maker Electronic Arts Inc said it was working with authorities to gather facts.
“This is a horrible situation, and our deepest sympathies go out to all involved,” the company said on Twitter.