YouTube is promoting conspiracy theory videos claiming that the Las Vegas mass shooting was a hoax, outraging survivors and victims’ families, in the latest case of tech companies spreading offensive propaganda.

It’s only been days since a gunman inside the Mandalay Bay hotel opened fire on a music festival, killing 58 people and injuring nearly 500. But videos questioning whether the shooting really happened and claiming that the government has lied about basic facts have already garnered millions of views on YouTube and are continuing to run rampant.

It appears YouTube is actively helping these videos reach wide audiences. Searching for “Las Vegas shooting videos” immediately leads to a wide range of viral videos suggesting that law enforcement and others have purposefully deceived the public. Some label the tragedy a “false flag”, a term conspiracy theorists typically use to refer to mass shootings they say are staged by the government to advance gun control.

Stephen Melanson, whose wife and daughter were both shot in the attack, told the Guardian he believed YouTube should take down videos suggesting the deadliest mass shooting in modern US history had been faked.

“When I see my wife fighting for her life with a gunshot wound to her chest, and my daughter was also shot, it’s pretty conclusive evidence that it did happen,” said Melanson, whose wife, two daughters and two friends escaped alive from the Route 91 Harvest festival on Sunday night. “My daughter texted me … ‘There is a shooting right in front of us’ and another text said, ‘Mom is shot.’”

One video on the first page of results on the Google-owned video platform Wednesday morning was called Las Vegas ‘Shooting’ … Did It Actually Happen? and questioned whether the attack was “fake” and if victims were “actors”. It had more than 250,000 views after one day on the site.

YouTube told the Guardian that this footage and other specific conspiracy videos that appeared after a generic search did not violate its standards.

Complaints about YouTube enabling fake news follow reports that within hours of the tragedy, both Facebook and Google were actively promoting rightwing blogs and conspiracy sites, some misidentifying the shooter and claiming he was a Democrat who opposed Donald Trump. The proliferation of politicized propaganda comes as Silicon Valley corporations are facing increased scrutiny over their role in allowing false news to reach millions on their platforms, possibly assisting Russia’s efforts to interfere in US politics.

Conspiracy theories about mass shootings are nothing new in America, but some fear YouTube’s popularity and algorithms are exacerbating the problem. In addition to automatically suggesting Las Vegas conspiracy footage to people with basic search requests, YouTube has promoted a wide variety of fake related content to people who watch a single propaganda video.

That means for some, YouTube is not suggesting reputable media sources and instead is exposing them to dozens of videos painting a picture of a vast conspiracy.

After the Guardian watched one questionable video highlighted on the main Las Vegas shooting search page (from a gun rights advocate suggesting the facts didn’t add up), YouTube promoted: “Government Staged Las Vegas Mass Shooting” (215,000 views), “PROOF: MEDIA & LAW ENFORCEMENT ARE LYING ABOUT THE VEGAS SHOOTING” (660,000 views) and “PROOF Las Vegas shooting FALSE FLAG hoax” (70,000 views). The site automatically played a “Las Vegas HOAX Exposed” video (150,000 views).

YouTube declined to comment on whether it has since removed footage, and it’s unclear if the site has taken any measures to slow the spread of Las Vegas fake news.

“It’s not a conspiracy. It’s not a joke. It did happen. I was there,” said Krista Metz, a witness and survivor who said she was standing close to the stage when the gunshots began. The 45-year-old California woman and her cousin were nearly trampled and ended up running for miles to escape, she said. “We literally thought we were going to die.”

Metz said she was frustrated with wild speculation online and in traditional news coverage: “People are so crazy with their social media. They’ll believe everything they read … It’s horrible.”

Even if YouTube and other sites remove the content, many have already viewed the videos – and many more will continue to publish similar messages, Metz noted. “There are so many people that post so many random things. How could they monitor it?”

“I just want the facts,” she added.

YouTube released a short statement touting its ability to promote “thousands of news publishers that present a variety of viewpoints”, adding, “When a major news event happens, these sources are presented on the YouTube homepage under ‘Breaking News’ and featured in search results, with the label ‘Top News’.”

“False flag” claims online can have devastating consequences for shooting survivors and families. The outspoken father of one of the children killed in the 2012 Sandy Hook elementary school shooting has faced frequent harassment and death threats from conspiracy theorists who believe the massacre was faked. One hoaxer who targeted him was recently sentenced to prison.

A woman who was badly injured in the Aurora theater shooting in Colorado also faced vicious online attacks and harassment, with critics trying to hack into her email account and accusing her of being a pawn in a gun control conspiracy.

Melanson – whose family was rescued by a retired firefighter and whose wife remains in the hospital following a second surgery – said he feared the propaganda on YouTube could impede law enforcement: “It’s hindering the investigation. They are creating false information that the authorities will still have to investigate. It really slows down the process.”

The videos also hurt victims and survivors already struggling to cope with trauma, he added: “It’s not fair to all the family members who have been going through this.”

Contact the author: Sam.Levin@theguardian.com