It happened after Sandy Hook, after Parkland, after the Orlando nightclub shooting and after the deadly rampage earlier this month at a Buffalo grocery store.

Within hours of Tuesday's school shooting in Uvalde, Texas, another rash began as internet users spread baseless claims about the man named as the gunman and his possible motives.

Unfounded claims the gunman was an immigrant living in the US illegally, or transgender, quickly emerged on Twitter, Reddit and other social media platforms.

They were accompanied by familiar conspiracy theories suggesting the entire shooting was somehow staged.

The claims reflect broader problems with racism and intolerance toward transgender people, and are an effort to blame the shooting on minority groups who already endure higher rates of online harassment and hate crimes, according to disinformation expert Jaime Longoria.

"It's a tactic that serves two purposes: It avoids real conversations about the issue (of gun violence), and it gives people who don't want to face reality a patsy, it gives them someone to blame," said Longoria, director of research at the Disinfo Defense League, a non-profit that works to fight racist misinformation.

In the hours after the shooting, posts falsely claiming the gunman was living in the country illegally went viral, with some users adding embellishments, including that he was "on the run from Border Patrol".

"He was an illegal alien wanted for murder from El Salvador," read one tweet liked and retweeted hundreds of times.

"This is blood on Biden's hands and should have never happened."

The man who authorities say carried out the shooting, 18-year-old Salvador Ramos, is a US citizen, Texas Governor Greg Abbott said in a news conference on Tuesday.

Other social media users seized on images of innocent internet users to falsely identify them as the gunman and claim he was transgender.

On the online message board 4Chan, users liberally shared photos and discussed a plan to label the gunman as transgender, without any evidence to back it up.

One post on Twitter, which has since been deleted, featured a photo of a trans woman holding a green bottle to her mouth, looking into the camera, headphones hanging from one ear.

"BREAKING NEWS: THE IDENTITY OF THE SHOOTER HAS BEEN REVEALED," claimed the user, saying the shooter was a "FEMBOY" with a channel on YouTube.

None of that was true. The photo actually depicted a 22-year-old trans woman named Sabrina who lives in New York City.

Sabrina, who requested her last name not be published due to privacy concerns, confirmed to The Associated Press that the photo was hers.

Sabrina said she received harassing responses on social media, particularly messages claiming that she was the shooter.

"This whole ordeal is just horrifying," Sabrina told The AP.

Authorities have released no information on the gunman's sexuality or gender identification.

Arizona Congressman Paul Gosar fit both unfounded claims about Ramos in a single now-deleted tweet that also misspelled the suspect's name.

"It's a transsexual leftist illegal alien named Salvatore Ramos," Gosar tweeted on Tuesday night.

His office did not return a message seeking comment.

In some cases, misinformation about mass shootings or other events are spread by well-intentioned social media users trying to be helpful.

In others, it can be people looking to start fake fundraisers or draw attention to their website or organisation.

Then there are the trolls who seemingly do it for fun.

Fringe online communities, including on 4chan, often use mass shootings and other tragedies as opportunities to sow chaos, troll the public and push harmful narratives, according to Ben Decker, founder and CEO of the digital investigations consultancy Memetica.

"It is very intentional and deliberate for them in celebrating these types of incidents to also influence what the mainstream conversations actually are," Decker said.

"There's a nihilistic desire to prove oneself in these types of communities by successfully trolling the public.

"So if you are able to spearhead a campaign that leads to an outcome like this, you're gaining increased sort of in-group credibility."

For the communities bearing the brunt of such vicious online attacks, though, the false blame stirs fears of further discrimination and violence.

Australian Associated Press