Witness Ryen Aleman was watching a game at the tournament in Jacksonville when he saw the shooting and heard a barrage of gunfire.
Aleman said he ducked to the ground and crawled to the restroom while everyone ran for safety.
He said the shooter may have had a laser to assist his aim on the gun.
"I’m still shocked, I can hardly talk," Aleman said.
Memorial Hospital in Jacksonville, Florida is treating at least three patients that were involved in the gaming shooting, hospital spokesman Pete Moberg tells CNN.
All of the patients are in stable condition, Moberg said.
Preliminary reports suggest at least 11 people were shot at the Jacksonville gaming tournament, according to a law enforcement official briefed on the incident.
Four of the people shot were killed, the source said.
Update: The number of people killed is 3, including the shooter, and 9 other people were shot. This initial post stated that 4 were killed and 7 wounded based off of preliminary information that was subject to change.
Electronic Arts — the digital interactive entertainment company that produces the Madden video game and that sanctioned Sunday’s Jacksonville tournament — issued a statement on its Twitter account.
"This is a horrible situation, and our deepest sympathies go out to all involved," the company tweeted.
Here's the full message:
Jason Lake, CompLexity Gaming founder and CEO, just issued a statement on the condition of one of their players, who was wounded during today's shooting at the Madden 19 Tournament.
The player was hit by gunfire and ran to a nearby gym for safety.
Read his full statement:
“We're obviously shocked and saddened by this afternoon's events. Our player, Drini, was hit in the thumb but is going to be fine. He managed to escape and run down the street to a nearby gym. He's currently cooperating with the authorities and we will be flying him out of Jacksonville as soon as we are given the green light from the officials on the ground.”
CNN has obtained a video capturing what appears to be the moment gunshots began to ring out while the tournament was being broadcast online.
The video captured the sound of approximately a dozen gunshots, and screams.
Warning this video may be disturbing to some viewers:
Florida Gov. Rick Scott says Florida Department of Law Enforcement personnel are responding to the Madden tournament shooting in Jacksonville.
He said he's in contact with the Jacksonville Sheriff's Office.
The Jacksonville Sheriff's Office said officers are finding many people in hiding throughout the Landing, the event area where the mass shooting happened.
The office urged anyone who is hiding to stay calm and stay in their hiding spots.
In a second tweet, the sheriff's office asked anyone in hiding to call 911.
Malik Brunson was at a nearby Hooters when the shooting occurred at the GLHF Game Bar in Jacksonville, Florida.
"I was in Hooters, all I heard was he got a gun and the shooting started," Brunson told CNN. "One of the men ran into hooters with the shot wounds -- they locked us in the restaurant."
He shared the following photo to Facebook: