A losing player opened fire on his fellow video-gamers at a Madden NFL 19 tournament Sunday in Florida, killing two people, authorities said — as the horror played out live online.

Just before the first of a dozen shots rang out at around 1:30 p.m., the camera caught a laser dot creeping up the torso of Eli Clayton, aka “Trueboy,” who was in the middle of a match at the GLHF Game Bar in the Jacksonville Landing open-air mall.

The camera cuts away from him as screams fill the air and one of the gamer’s controllers abruptly disconnects.

“Oh f–k! What did he shoot me with?!” one victim yells between shots in the clip, as the carnage unfolds out of frame.

By the time gunman David Katz, 24, of Baltimore, ended his rampage, two people were dead — including Clayton — and another 11 were hurt, nine with gunshot wounds, according to local TV station News4Jax.

The other fatality was Taylor Robertson, 27, of Ballard, W. Va., according to The Miami Herald.

The gunman took his own life afterward, cops said.

Katz snapped and began targeting his rivals after being defeated earlier in the tournament, fellow gamer Steven “Steveyj” Javaurski told the Los Angeles Times.

“[He was here] all weekend,” said another gamer, who spoke to WJAX-TV and claimed to have beaten Katz on Saturday.

“He had shades on, he didn’t speak to anybody,” the man said.

“Even after we played . . . I went to shake his hand and just tell him good game and he just looked at me. He didn’t say anything.”

Gamers told WJAX that Katz was defeated by two players during the two-day tournament, both of whom survived the shooting.

Katz reportedly wore the same clothing on both days.

“I mean, obviously you don’t need a change of clothes if you already were preparing to do something like that,” said the gamer who claimed to have beaten Katz.

Katz reportedly used a semiautomatic handgun in the assault.

The Jacksonville Sheriff’s Office warned people on Twitter to keep out of the area while the mayhem was unfolding.

“Mass shooting at the Jacksonville Landing. Stay far away from the area,” the office tweeted at 2:13 p.m. “The area is not safe at this time. STAY AWAY.”

Authorities set up a three-block perimeter around the riverfront mall, clearing the way for SWAT teams, which entered the blood-drenched gaming bar and pizzeria, where terrified survivors were hunkered down.

“We will get to you,” another tweet from cops read. “Please don’t come running out.”

Once first responders confirmed that the shooter was dead, they turned to triaging the other victims, whom officials didn’t immediately name.

At least two other gamers were reported among the wounded.

“The tourney just got shot up. Im leavinng [sic] and never coming back,” tweeted 19-year-old competitor Drini “Young Drini” Gjoka.

“I am literally so lucky. The bullet hit my thumb. Worst day of my life,” he added.

Fellow competitor Timothy “oLARRY” Anselimo was also among the injured, according to his mother.

“My son was shot 3 times,” tweeted Sujeil Lopez. “Please keep him in your prayers.”

Jason Lake, the founder and CEO of CompLexity Gaming, for which Gjoka plays, told CNN, “To have someone walk into an event like this that’s all about good sportsmanship and teamwork and just good vibes, and do something like this, it’s heartbreaking.

“I think that the e-sports industry as a whole is going to have to step back and take a look at further strengthening our security.”

The sanctioned tournament for this year’s installment of the long-running Madden NFL video-game franchise offered $5,000 in prizes, with the top two finishers qualifying for spots in the Madden Classic Live Finals in Las Vegas.

The gamer who beat Katz on Saturday told WJAX that the gunman was “a well-known competitor” who played across the country.

Video footage from last year shows Katz gaming and talking trash at a Madden tournament in Buffalo sponsored by the Bills. He won that tourney, despite being the seventh seed.

“I don’t think of myself as a seventh seed,” boasted Katz, aka “Bread,” following a 41-7 win over a No. 2 seed.

“I think personally I’m one of the better players — and I like to let my game prove that.”

On Sunday, Katz was introduced by Madden announcers as “a man of business” who “keeps to himself.”

“You are not going to see much emotion,” an announcer said. “He’s not here to make friends.”

Additional reporting by Ben Feuerherd, Nolan Hicks and Wires