Here's what we know so far:
>> On ActionNewsJax.com: PHOTOS: Victims of Jacksonville Landing shooting
Eli Clayton, 22
Eli Clayton, 22, of Woodland Hills, California, was a talented "Madden 19" player who enjoyed success using the Jacksonville Jaguars in tournaments, according to ActionNewsJax. He was recognized by the team in 2017 for taking its digital likeness to the national finals that year, the news station reported.
A haunting video from Sunday's tournament in Jacksonville showed what appeared to be a gun's laser point on his chest before he was shot and killed, the Miami Herald reported.
Known in gaming circles as "True" and "Trueboy," Clayton was "consistently one of the best in competitive 'Madden,'" his EA Sports profile says. He reportedly took home $51,000 in prizes in head-to-head live events, winning 40 percent of his 20 games.
"Despite a rough season, True remains one of the best around and can potentially win it all when it's done," the EA Sports website says.
In an April article by EA Sports, Clayton described himself as "really easy to get along with."
"I'm not a troublemaker," he said. "I'm always laughing and joking around. I'm just me, a cool dude. There's really nowhere to go but up, honestly."
Competitive gamer Shay "Young Kiv" Kivlen of Seattle called Clayton "one of my best friends in life" in a Twitter tribute Sunday.
"I talked to u almost everyday for the last 5 years," he wrote. "U were one of the most kind and genuine people I've ever met. I love u like a brother. I'm gonna miss hearing you laugh everyday and seeing your genuine smile."
>> See the tweet here
Taylor Robertson, 27
Taylor Robertson, a 27-year-old father from Ballard, West Virginia, also was killed in Sunday's mass shooting, ActionNewsJax reports. He is survived by his wife and young son, according to the Metro News.
Robertson, known in the gaming world as "SpotMePlzzz," had "the skill to completely take over any 'Madden' tournament," his EA Sports profile says. He reportedly took home $80,500 in prizes in head-to-head live events, winning 72 percent of his 18 games.
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"His potent offensive talent paired with his skill on defense makes him one of the toughest opponents in competitive 'Madden,'" the profile says.
Members of the "Madden" community said Robertson competed to help support his family.
"Always spoke about his wife & newborn and how he wanted the money to better their lives," @EricRayweather tweeted.
>> See the tweet here
– ActionNewsJax contributed to this report.