Heavily armed agents have entered a Baltimore townhouse, the home of the father of the suspect in a deadly shooting at a video game tournament in Jacksonville, Florida.

Jacksonville Sheriff Mike Williams said investigators believe 24-year-old David Katz of Baltimore was the shooter in an attack Sunday afternoon that left three people dead, including the suspect with a self-inflicted gunshot wound. Eleven others were reported injured.

An FBI spokesman, Dave Fitz, confirmed agents had gone to the upscale townhouse home. But he did not immediately release any other specifics, citing an ongoing investigation.

Police have raided the home of the father of the suspect behind a deadly Florida shooting. (AAP)

US police have confirmed the shooting suspect as 24-year-old David Katz. (YouTube) (YouTube)

Several people are dead and "many" taken to hospital after a shooter opened fire at a riverside marketplace in downtown Jacksonville, Florida. (Facebook/Malik Brunson)

The agents could be seen inside the home on Sunday evening near Baltimore's Inner Harbor. They wore bulletproof vests and carried long guns. Reporters who gathered outside did not see any authorities carrying any bags of evidence outside.

Witnesses have described the terrifying moment a video gamer killed two people and wounded several others when he opened fire with a handgun at a tournament that was being streamed online from a restaurant.

Sheriff Williams declined to comment on what led to the third major mass shooting to hit Florida in the last two years.

Williams said Katz killed himself after the shooting and his body was found along with those of his two victims.

Jacksonville Sheriff Mike Williams has described the suspect as a white man, found dead at the scene. (AP)

Australia's Department of Foreign Affairs and Trade said in a statement it was "not aware of any Australians being affected by the shooting".

Dozens of ambulances and police cars flooded into The Jacksonville Landing, a waterfront dining, entertainment and shopping site in the city's downtown, after several shots rang out on a sunny Sunday afternoon local time.

The shooting took place during a regional qualifier for the Madden 19 online game tournament at the GLHF Game Bar inside a Chicago Pizza restaurant, according to the venue's website.

(9news)

The bar was livestreaming the football video game competition when the gunfire started, according to video of the stream shared on social media.

In the video, players can be seen reacting to the shots and cries can be heard before the footage cuts off.

Taylor Poindexter and her boyfriend, Marquis Williams, had travelled from Chicago to attend the tournament, and they fled when the gunfire erupted. She said she saw Katz take aim at his victims.

The shooting occurred at a riverfront mall in Jacksonville. (AP)

"We did see him, two hands on the gun, walking back, just popping rounds," Poindexter told reporters. "I was scared for my life and my boyfriend's."

One Twitter user, Drini Gjoka, said he was taking part in the tournament and was shot in the thumb.

"Worst day of my life," Gjoka wrote on Twitter. "I will never take anything for granted ever again. Life can be cut short in a second."

Local media said the shooter had been competing in the tournament and lost, then apparently targeted other players before killing himself.

The latest rampage occurred amid a debate over US gun laws that was given fresh impetus by the massacre in February of 17 people at a high school in Parkland, Florida.

A still from the Madden 19 Tournamentt livestream in which shots were heard. (Madden 19)

The video game view was replaced by a "controller disconnected" message shortly after the shooting began. (Twitch)

Two years ago a gunman killed 49 people at the Pulse nightclub in Orlando.

The sheriff's office said many people were transported to hospital, and its deputies found many others hiding in locked areas at The Landing.

Florida Governor Rick Scott said he had offered to provide local authorities with any state resources they might need.

Police have barricaded a street near Jacksonville Landing. (AP)

"Word of another tragic mass shooting in our state brings shock and outrage," Nelson said on Twitter.

President Donald Trump was been briefed and was monitoring the situation in Jacksonville, the White House said.

Calls have been made for greater security at similar events.

Joey Cuellar, the tournament director for the Evolution Championship Series, an esports event that focuses on fighting games tweeted: "It's very clear that we need to be more proactive for 2019 and beyond."

The tournament is held in Las Vegas and draws some 15,000 people. In March, organizers called the FBI when someone wrote online: "mass shooting @EVO18 see you there."

The event went off without a hitch, but Cuellar also wrote on Sunday: "The amount of undercover law enforcement at Evo was unprecedented, and we will be installing metal detectors for ALL days next year."

Esports are big business. A Goldman Sachs report in 2017 valued eSports at $500 million in 2016 and anticipated market growth.

At Sunday's Madden competition, the tournament was streamed live on Twitch.tv, an online network that attracts tens of millions of visitors, most of whom watch footage of other people playing video games.