He said Katz was in Jacksonville for the "Madden NFL 19" video game tournament, at Jacksonville Landing, in the heart of the city's downtown.
In the Madden community, he had played 18 games and won 72 percent of them.
A tweet by National Football League team Buffalo Bills two years ago shows a picture of a man named David Katz holding up a t-shirt that says "Madden champ", with the caption: "Congrats to David Katz, the Madden 17 Bills Championship victor!"
Video game players participating in the tournament also took to social media to share the details of the incident as it unfolded.
William said Katz committed the shooting with a single handgun.
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'Madden NFL 19' Livestream Tournament Shot Up in Jacksonville, FL
Sheriff Mike Williams provided additional details hours after a gunman opened fire at a Jacksonville, Florida, mall on Sunday. We ask you to stay calm, stay where you are hiding. "Please don't come running out".
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Earlier today, the Jacksonville Sheriff's Office identified the alleged shooter as a white male who had died at the scene but declined to give his name or where he was from.
"One of [the guns Katz used] did have like an after market laser sight that attaches to the trigger guard", Williams added.
"They were great competitors and well-loved members of the Madden community".
Williams said the FBI is assisting in the ongoing investigation, including looking further in the suspect's background. "David Katz keeps to himself. He's not here to make friends", an announcer said during a YouTube livestream of the 2017 tournament, as pointed out by the Business Insider. "You can't even get him to open up about anything, it's like pulling teeth". A rival of the shooter recalled that the 24-year-old didn't talk much with fellow gamers, either online or when they met face-to-face for tournaments.
According to the San Diego Union-Tribune a shooter opened fire during a Madden video game tournament in Jacksonville Sunday afternoon killing four people and injuring nearly a dozen others. He scores a touchdown, and then the feed switches to a kickoff return - and 11 gunshots can be heard. We have made a decision to cancel our three remaining Madden Classic qualifier events while we run a comprehensive review of safety protocols for competitors and spectators.