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JACKSONVILLE (CBSMiami/AP) – A gunman opened fire Sunday at an online video game tournament that was being livestreamed from a Jacksonville, Florida mall, killing multiple people and sending many others to hospitals, authorities said.

David Katz, 24, of Baltimore, Maryland, is believed to be the shooter and died from a self-inflicted gunshot wound, Jacksonville Sheriff Mike Williams said, describing him as a white male who was attending the gaming event. Williams said he did not know if Katz knew the victims.

The shooting took place at The Jacksonville Landing in the city’s downtown area. In a brief press conference, Williams said police had cleared the scene and were not looking for other suspects. He did not say how many people were killed or injured in the shooting.

The sheriff’s office had earlier said there were “multiple fatalities” and that many people had been transported to local hospitals. FBI agents are at the scene to assist the sheriff’s office, which is leading the investigation, a law enforcement official told CBS News.

Doctors at UF Health Jacksonville said they were treating six victims. Five were listed in good condition. Another victim, who suffered a gunshot wound to the chest, was listed in critical but stable condition. Memorial Hospital said it was treating three patients, who were in stable condition.

Sheriff Williams said authorities had yet to identify the suspect who attacked the video football tournament, which featured the game “Madden NFL 19.” The competition was held in a gaming bar that shares space with a pizzeria. Viewers could watch the games online and see the players.

Investigators were looking into online video that appeared to capture the scene right before the shooting began, Williams said.

A red dot that appears to be a laser pointer is visible on the chest of a player seconds before the first of a dozen gunshots rings out.

The sheriff’s office used Twitter and Facebook to warn people to stay far away and to ask anyone who was hiding to call 911.

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. #TheLandingMassShooting https://t.co/qBJvkaO7xT — Jax Sheriff’s Office (@JSOPIO) August 26, 2018

“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 sheriff’s office said via Twitter.

The sheriff’s office did not provide any other information, but also warned reporters to stay away from the area.

Police barricaded a three-block radius around the mall. Officers and Coast Guard boats patrolled the nearby river. Many ambulances could be seen in the area, but the mall area appeared empty of all but law enforcement. Police also took up positions on a bridge overlooking the river.

On an online stream of the Madden event posted to the website Twitch, several loud gunshots can be heard and the game abruptly stops.

After several gunshots, people scream and one person cries out, “Oh f**, what’d he shoot me with?” The stream did not show the shooting.

Warning: You can watch/listen to part of that stream by clicking here, but the audio is very graphic.

Drini Gjoka, who was competing in the event, tweeted about the shooting in the immediate aftermath. “The tourney just got shot up. Im leavinng and never coming back,” he wrote at 1:37 p.m. Four minutes later, he added that a bullet struck him in the thumb, and later called Sunday the “worst day of my life.”

“I will never take anything for granted ever again. Life can be cut short in a second,” he said.

Florida Gov. Rick Scott, who was traveling to Jacksonville, tweeted that he offered state resources to the local law enforcement. His office said the Florida Department of Law Enforcement and Florida Highway Patrol were assisting in the investigation.

EA Sports said it was aware of the shooting and is working with authorities to gather information. “This is a horrible situation, and our deepest sympathies go out to all involved,” the company said.

The NFL issued a statement saying it was “shocked and deeply saddened by the horrific tragedy” in Jacksonville. “We support our partners at EA Sports and will continue to monitor developments with local law enforcement.”

The Jacksonville Landing, in the heart of the city’s downtown, has many bars and restaurants and also hosts concerts and other entertainment. It was the site of a Donald Trump rally in 2015, early in his campaign for the White House.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)