A witness to the deadly Charlottesville attack at a white supremacist rally has sued Alex Jones and his site Infowars, alleging that the rightwing commentator spread defamatory conspiracy theories that led to a torrent of digital abuse, in-person harassment and death threats that have persisted for months.

Brennan Gilmore, a counter-protester at the “Unite the Right” rally who had captured footage of the car attack that killed Heather Heyer, filed the lawsuit on Tuesday against Jones, Jim Hoft of the pro-Trump conspiracy site Gateway Pundit, and five others. After Gilmore posted video of the attack last August and spoke to the media, he quickly became the subject of viral conspiracy narratives falsely claiming he was a CIA or “deep state” agent who helped orchestrate the violence as part of a government operation.

Jones, the host of Infowars, in Texas. Photograph: Tamir Kalifa/AP

“I naively wasn’t expecting any backlash from doing these interviews and condemning something that I thought we would universally condemn – the resurgence of Nazis,” Gilmore told the Guardian. “It was just sort of a full flood of hate mail, death threats and harassment.”

Georgetown Law’s civil rights clinic has filed the suit at a time of growing concern about conspiracy theories surrounding major US events, which have proliferated on social media and led to widespread abuse of survivors of mass shootings and families of those killed in tragedies. There have also been increasing stories of real-world consequences, including a 2016 shooting inspired by the “Pizzagate” conspiracy theory about Hillary Clinton and a pizza shop, and more recently, harassment at a church in a small Texas town that suffered one of the deadliest mass shootings in modern US history.

Infowars and Gateway Pundit have repeatedly faced accusations of spreading lies and producing content that leads to massive harassment campaigns.

Gilmore, who is a musician and has worked as a foreign service officer for the US state department, said he had publicized his footage and spoken to reporters in hopes of setting the record straight. He said the conspiracy theory posts and abuse began immediately, including some “alt-right” people publicizing the address of his parents, who live in the region.

In addition to violent online threats, Gilmore said he received physical hate mail, including a package with a powdery substance. He said some conspiracy theorists have confronted him on the street and that even though months have passed, someone recently threatened that his body would end up in a local river.

“It has fundamentally changed my life,” said Gilmore, who noted that someone has also threatened to show up to one of his band’s shows. “I’m incredibly concerned about the capacity for this to happen to other people and the danger this poses to our country.”

Some content targeting Gilmore was headlined “State Department/CIA Orchestrated Charlottesville Tragedy” and “Random Man at Protests Interviewed by MSNBC, NY Times is Deep State Shill”. Allen West, a former Florida congressman and Fox News contributor who was also named in the lawsuit, had published an article called, “BOMBSHELL: New evidence suggests Charlottesville was a complete SET-UP”.

Andrew Mendrala, supervising attorney of the Georgetown law clinic, said Gilmore had a strong defamation case given the harm he suffered, adding that he hoped the suit would help prevent these kinds of damaging conspiracy theories.

Charlottesville, Virginia, after dozens were injured in clashes between white supremacists and counter-protesters. Photograph: Michael N/Pacific/BarcroftImages

“The first amendment does not protect this type of speech. We think a victory would really protect people like Brennan in the future,” Mendrala said in an interview.

Gilmore said sites such as YouTube, Twitter and Facebook should do a better job limiting the spread of malicious content, noting they were slow to respond to his complaints about harassment.

“These social media companies unleashed an incredibly potent force, and it’s one that can be used for good and one that can be used for bad,” he said, adding: “They can’t quite keep up.”

YouTube in particular has faced intensifying scrutiny for its algorithms promoting conspiracy theories, fake news and far-right propaganda in recent years.

Gilmore said he was aware that speaking out could invite further abuse and that he has been forced to consult security experts about how to protect himself and his family: “Despite the personal risk involved, this is incredibly important to pursue.”

Infowars, Jones and West did not respond to requests for comment. Hoft said in an email that the lawsuit was “bullshit” and “full of lies”, adding, “There is no case.” He did not elaborate, but published a response on Gateway Pundit on Tuesday that called Gilmore an “unhinged leftie hack” and repeated many of the conspiracy claims.