"The Department also tweeted that "we are finding many people hiding in locked areas at The Landing", asking those individuals to "stay calm" and remain hidden". Shots can be heard in video from the live stream, which eventually cuts off amid shouts and gunfire.
" In a video circulating social media, an announcer is heard introducing Katz by commenting on his intense focus and standoffish attitude".
"This is a terrible situation, and our deepest sympathies go out to all involved", Madden's creator, EA Sports, said in a statement.
One Twitter user, Drini Gjoka, said he was in the tournament and was shot in the thumb.
"Searches are being conducted", the sheriff's office said. "Love you all and hope something like this never happens again".
Shortly after 1:30 p.m. ANAKRON tweeted "madden tournament getting shot up on twitch".
Another gamer, Chris "Dubby" McFarland, was hospitalized after a bullet grazed his head. "I feel fine, just a scratch on my head. Traumatised and devastated", he tweeted.
The killings rocked the world of professional electronic gaming, also known as esports, which boasts an estimated 250 million players worldwide in a growing market worth about a billion dollars a year.
Katz was in Jacksonville for the "Madden NFL 19" video game tournament, authorities said.
TRT World's Steve Mort has more from Florida.
Law enforcement officials told local NBC News affiliate First Coast News that four people are dead and 11 are injured. He said the FBI was searching the man's home as part of the investigation.
The shooting occurred at the Chicago Pizza restaurant at Jacksonville Landing, authorities said Sunday evening. Florida Gov. Rick Scott said Trump called him and offered any federal help he needed.
White House press secretary Sarah Sanders confirmed President Donald Trump had been briefed on the incident. "It will require us to continue to do the hard work of public safety to make sure people are safe", Curry said.
A Madden 19 football tournament was scheduled to take place Saturday and Sunday at GLHF Game Bar, which is located at Jacksonville Landing.