JACKSONVILLE, Fla. — A Baltimore man has been identified as the gunman who opened fire Sunday at a video game tournament at a mall in Jacksonville, Fla.

Jacksonville Sheriff Mike Williams said Sunday evening that he believed the shooter to be 24-year-old David Katz of Baltimore. He said the FBI was searching the man’s home as part of the investigation.

The sheriff said three people were killed, including the shoooter, and nine other people were wounded by gunfire and are in stable condition. Two others were hurt while fleeing the shots.

Williams said Katz committed the shooting with a single handgun. He said Katz was in Jacksonville for the “Madden NFL 19” video game tournament. The games maker, EA Sports, lists a David Katz as a 2017 championship winner.

The shooting was partially captured on a livestream of the “Madden NFL” gaming tournament at the Jacksonville Landing, a downtown marketplace.

In video captured from the livestreaming network Twitch and posted on social media, shots can be heard as footage of a kickoff return is replaced by the message “controller disconnected.”

The shooter was a gamer who was competing in the tournament and lost, according to Stephen “Steveyj” Javaruski, one of the competitors.

The shooter “targeted a few people” and shot at least five victims before killing himself, Javaruski told the Los Angeles Times in a direct message on Twitter. The gunman killed two or three people “that I saw,” Javaruski said.

In a public tweet, he added that he was escorted out by police after the shooting. “I am literally so lucky,” gamer Drini Gjoka said in a tweet. “The bullet hit my thumb.”

A Chicago man said people trampled each other while fleeing the gunfire. Twenty-year-old Marquis Williams said he and his girlfriend, Taylor Poindexter, also of Chicago, were ordering pizza when shots erupted. He said, “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.”

Williams had participated earlier Sunday in the gaming tournament. He said he got a view of the back of the shooter in a green shirt.

He described people running in the panic to escape. He said it was then that he scuffed his leg and that Poindexter was treated at a hospital for a sprained ankle.

Federal authorities were at a home in the 1200 block of Harbor Island Walk near Baltimore’s Inner Harbor on Sunday night as part of an investigation into the Jacksonville mass shooting.

Baltimore Police spokesman T.J. Smith said in an email that “we are assisting our partner law enforcement agencies with some information that has led authorities to Baltimore.”

Matt Munoz and Cameron Stearns were eating dinner next door when they saw agents with rifles walking around the back. “It’s like, ‘Uh, what’s going on?’?” Munoz said.

They were briefly interviewed by agents, who asked among other things if ammunition had ever been mistakenly delivered to their home.

Stearns, 33, said he had seen Katz but never spoken to him beyond cordial greetings. “I definitely had seen him coming and going from the house. The FBI guy showed me his picture, and I was like, that’s definitely him,” Stearns said. “When he was younger he definitely lived here. Now, I’m not so sure. I hadn’t seen him as frequently.”

The police activity in the neighborhood was “shocking, to say the least,” Stearns said, adding that the area is very quiet and rarely sees even petty crimes.

Earlier Sunday, people in the online gaming community had identified Katz as the shooter, and said he had played under the game tag “RavensChamp” and “Bread.”

Video game tournaments have been growing in recent years, with winners competing for thousands of dollars and other prizes. One of the tournaments Katz won last year had play-by-play commentators, and he was interviewed by former NFL player and broadcaster Steve Tasker, according to a YouTube video.

“Personally I think I’m one of the better players and I like to let my game prove that,” Katz told Tasker after defeating a higher seed.

The weekend competition was a regional qualifying round for the Madden NFL Championship series, a national tournament.

Sunday’s shooting appeared to unfold during the second round of play. By 4:30 p.m., investigators had cleared the shooting scene and were still questioning witnesses, the sheriff said at a briefing. Investigators were reviewing video of the shooting posted on social media and encouraged those with access to such footage to contact them, Williams said.

Six victims — ranging in age from 20 to 35 — were taken to University of Florida Health Jacksonville hospital, according to hospital spokesman Dan Leveton, who said that five were in good condition, one in serious condition. He said that most suffered multiple gunshot wounds and that one who was hit in the chest was expected to stay overnight for surgery.

Three other victims were taken to Memorial Hospital in Jacksonville and were in stable condition, spokesman Peter Moberg said.

Baltimore Sun reporter Christina Tkacik and the Associated Press contributed to this article.