Police said they believed David Katz, of Baltimore, was in Jacksonville for the Madden NFL 19 event on Sunday and carried out the shooting before he was found dead from a self-inflicted gunshot.
Unconfirmed reports said he had been eliminated from the competition at The Landing shopping centre before the shooting, in which nine other people were injured.
A man called David Katz won a similar event last February. Madden NFL’s publisher, EA Sports, lists him as a 2017 tournament winner and the official Twitter account of the Buffalo Bills, the football team, posted a picture of Katz congratulating him on his victory.
Jacksonville Sheriff Mike Williams said authorities believe Katz carried out the attack using at least one handgun at the Jacksonville Landing, a collection of restaurants and shops along the St Johns River. Williams said final confirmation of the suspect’s identity was still pending.
The shooting took place during a regional qualifier for the Madden 19 online game tournament at the GLHF Game Bar inside a pizza restaurant, according to the venue’s website.
The winners of the contest go forward to the video game finals in Las Vegas and compete for $165,000 (£129,000).
Questions are being asked about the level of security provision at the tournament. One participant said he had not seen any security screening at the entrance, and suggested such considerations had failed to keep with the seriousness and scale of such events.
Late on Sunday local time, heavily armed officers entered a Baltimore home where Katz was believed to have lived.
The agents could be seen inside the home Sunday evening near Baltimore’s Inner Harbour. They wore bulletproof vests and carried long guns. Reporters who gathered outside did not see any authorities carrying any bags of evidence outside.
An FBI spokesman, Dave Fitz, confirmed agents had gone to the upscale townhouse home. But he did not immediately release any other specifics, citing an ongoing investigation.
Witnesses described people trampling on each other as they raced to flee the scene of Sunday’s shooting, the sounds of which were captured as the gaming tournament was being livestreamed.
In one video, a red laser sight can be seen tracing across the chest of a man who is playing the game. The video cuts out, and the sounds of gunshots can be heard before the livestream ends altogether.
Twenty-year-old Marquis Williams said he and his girlfriend, Taylor Poindexter, both of Chicago, were ordering pizza close by when shots erupted. He said he didn’t think it was gunfire at first.
“Initially we thought it was a balloon popping, but there weren’t any balloons in the room. Then we heard repeat shots and we took off running,” he said.
Mr Williams had participated in the gaming tournament earlier, and said he got a view of the back of the shooter in a green shirt.
He described people running in the panic to escape, and said he scuffed his leg and that Ms Poindexter was treated at hospital for a sprained ankle. They later returned, hoping to retrieve her cell phone, but the scene was cordoned off by authorities.
Madden competitor Derek Jones, 30, of Santa Fe, New Mexico, said: “No one deserves to die over playing a video game, you know? We’re just out here trying to win some money for our families and stuff.”
Mr Jones said he was sitting in a back patio outside the tournament venue when he heard the gunshots. He jumped a fence and ran, leaving behind his backpack and cellphone.
“You know, I’m glad I lost today,” Mr Jones said. “Because if I’d won, I would have been in that game bar right then playing a game and not paying attention. And he could have come and I’d probably be dead right now.”
Mr Jones said he knew Katz by the gamer tags he used online – often “Bread” or “Sliced Bread” – and had played against him online but had never spoken to him personally.
Florida Governor Rick Scott said President Donald Trump has called him to offer federal support. His spokesman, John Tupps, said the governor had also spoken on Sunday with Jacksonville Mayor Lenny Curry and the sheriff, Mike Williams.
The governor was preparing to fly from his home in Naples to Jacksonville and meet with investigators, Mr Tupps said. The Florida Department of Law Enforcement and Florida Highway Patrol have also been directed to help with the investigation.
Additional reporting by agencies