When two people were killed and 11 injured at a Madden football video game tournament at a bar in Jacksonville, Florida, on Sunday, the incident joined the list of hundreds of tragic shootings that have already taken place in the US this year. But it was the first to occur during a video game tournament, and the first to have been unintentionally broadcast live online.

Viewers were watching the tournament online via Twitch, a video-streaming platform. Some of the millions of people who tune in daily had suddenly found themselves witnesses to a violent crime. In the disturbing footage, which has since been taken down by Twitch, a laser sight could be seen on the chest of one of the competitors before the first shots were fired. As news of the shooting and its first moments of live broadcast hit the media, it was the first time many non-gamers had heard of Twitch.

But at any given time, nearly a million people are watching Twitch, spread out across thousands of different channels. The vast majority of Twitch’s broadcasters are playing video games in front of an audience, ranging from gigantically popular multiplayer games like Fortnite and Counter-Strike to retro obscurities from the 1990s. Some individual Twitch broadcasters reach hundreds of thousands of viewers; others stream for an audience of fewer than 10 people at a time.

You can technically livestream anything on Twitch. Its IRL (“in real life”) section features people cooking, drawing, dancing or just chatting. But Twitch gained popularity earlier this decade as a platform for watching people play video games, and it’s still the place where gamers congregate. When a big event happens in the gaming world, whether it’s a tournament or a news event like the Electronic Entertainment Expo in E3, in Los Angeles, where developers hype up their upcoming games, it is always broadcast on Twitch.

What makes Twitch different from traditional broadcasting is that it’s interactive. When watching a streamer, viewers can type into a chat window to communicate with each other and the streamer themselves, asking questions, making jokes or offering advice about the game. Successful streamers make their viewers feel like they’re in the living room with them, a parasocial relationship that nonetheless creates a feeling of belonging and togetherness for their audiences. Twitch’s format is extremely good at cultivating community, a virtual hangout spot for its millions of teenage and college-age users.

When it comes to eSports – professional competitive gaming, involving cash prizes and internationally known players – many thousands more people watch tournaments on Twitch than can travel to the real-world locations. The International, a huge tournament for the game DOTA 2 with a prize pool of $25m, had a peak concurrent viewership of 736,ooo on Twitch this year. Sunday’s tournament was a qualifier for the Madden championships in Las Vegas, with a much smaller viewership – it is not known exactly how many people were watching online when the shooting took place.

The Jacksonville shooting has been felt keenly in the online gaming community, and among Madden players especially. The shooter was a successful tournament Madden player. The two victims, Taylor Robertson and Elijah Clayton, made a living playing the game; Robertson had won the Madden Classic series in 2017. These were well-known people in the community, on and offline, as shown by the many tributes that have been posted on social media.

Livestreaming services like Twitch allow millions of people to feel like part of a community with favourite streamers, and watch admired players compete in game tournaments. But Sunday’s shooting wasn’t the first time such platform popularity took an unfortunate turn.

The easy access and interactive communications guarantee an expanded audience for malicious actors. “Swatting”, a dangerous prank involving calling armed police to a victim’s home on the basis of a fake tip-off, has been prevalent on Twitch for years; one man was killed by a Swat team in 2017 after another streamer called in a false hostage situation. Trolls sometimes attempt to get pizzas delivered to streamers’ homes live on stream, an act of intimidation designed to show the streamer in question that their home address is known.

Should a shooter decide to target an eSports tournament, they would be doing so in full knowledge that their actions would be broadcast to thousands outside of the arena itself. Sunday’s shooting has prompted calls for better security around these events. EA, publisher of the Madden video game series, has cancelled its remaining three qualifiers. Meanwhile, the organisers of the popular fighting game Evolution Championship Series have pledged to increase security, and Twitch itself issued a statement outlining increased security at October’s TwitchCon in San Jose, California, a convention for streamers and their fans.

Tuning into Twitch is something that millions of people do every day without a second thought. That something so horrific could happen at a tournament, unintentionally broadcast whilst viewers asked what was going on in the chat, has shocked players, viewers and the company alike.