The FBI monitored a prominent anti-war website for years, in part because agents mistakenly believed it had threatened to hack the bureau’s own site.

Internal documents show that the FBI’s monitoring of antiwar.com, a news and commentary website critical of US foreign policy, was sparked in significant measure by a judgment that it had threatened to “hack the FBI website” and involved a formal assessment of the “threat” the site posed to US national security.

But antiwar.com never threatened to hack the FBI website. Heavily redacted FBI documents, obtained through the Freedom of Information Act and shared with the Guardian, show that Eric Garris, the site’s managing editor, passed along to the bureau a threat he received against his own website.

Months later, the bureau characterized antiwar.com as a potential perpetrator of a cyberattack against the bureau’s website – a rudimentary error that persisted for years in an FBI file on the website. The mistake appears to have been a pillar of the FBI’s reasoning for monitoring a site that is protected by the first amendment’s free-speech guarantees.

“The improper investigation led to Garris and Raimondo being flagged in other documents, and is based on inappropriate targeting and sloppy intelligence work the FBI relied on in its initial memo,” said Julia Mass, an attorney with the ACLU of northern California, which filed the Freedom of Information Act request, and shared the documents with the Guardian.

FBI spokesman Paul Bresson said the bureau could not comment, as the ACLU’s litigation of the antiwar.com case is ongoing.

On 12 September 2001, Garris received an email with the subject line “YOUR SITE IS GOING DOWN.”

“Be warned assholes, ill be posting your site address to all the hack boards tonight, telling them about the little article at the moscowtimes and all. YOUR SITE IS HISTORY,” the unredacted parts of the email read.

Concerned, Garris forwarded the threatening email to the FBI field office in San Francisco, where he lives. (It is contained in the disclosed FBI documents.) “It was a threat and I wanted to report it,” Garris said.

But by 7 January 2002, someone in the field office characterized the message as “A THREAT BY GARRIS TO HACK FBI WEBSITE.”

According to unredacted portions of the documents, that apparent mix-up was the first time antiwar.com came onto the FBI’s radar – a purview that would last at least six years.

Garris said he never heard back from the FBI, and had no reason to believe that the incident had any broader impact, until he saw what was in his FBI file. “It was pretty scary to think that in my FBI file, and perhaps other government agency files, there was a report that I was considered a threat based on that,” Garris said.

“That may follow me for the rest of my life. Any time I interact with any law enforcement or government agencies, they’re going to be able to see that, and make evaluations of me based on it. It’s very scary.”

The mix-up did not stay limited to the San Francisco field office. In 2004, FBI officials in Newark, New Jersey, compiled a “threat assessment” of Garris and his colleague Justin Raimondo.

The 2004 “threat assessment” took note of Raimondo and Garris’ anti-war views, listed articles and media appearances in which they made their arguments, and reported that “peaceful” anti-war protesters in the UK passed out literature referencing antiwar.com that suggested the US government was holding Israeli spies in connection to the September 11 attacks.

Contained within the file are copies of articles penned by Raimando advancing the conspiracy theory that Israel had advance knowledge of the September 11 attacks.

The FBI created profiles of both Raimondo and Garris using public records, such as those held by the Department of Motor Vehicles, to include their physical descriptions. “The rights of individuals to post information and to express personal views on the Internet should be honored and protected; however, some material that is circulated on the internet can compromise current active FBI investigations,” reads an unsigned “analyst comment” on the 2004 threat assessment.

The unnamed analyst said there were “several unanswered questions regarding www.antiwar.com,” such as the source of its funding, the presence of material on the site that is “not suitable for public release” and whether Raimondo was using “a pseudonym.”

The redactions make it difficult to understand the legal predicates the FBI possessed for monitoring a website devoted to news and commentary. The revealed portions list only two potential possibilities. First is an “untitled spreadsheet” that the site posted, which apparently was linked to a “post-9/11 watch list [that] … may contain the names of individuals of active investigative interest.”

Yet the “analyst comment” on the threat assessment dismisses its importance: “The discovery of two detailed Excel spreadsheets posted on www.antiwar.com may not be significant by itself since distribution of the information on such lists are wide spread.” Raimondo has said that he found the document elsewhere on the internet.

The analyst listed a second rationale for monitoring antiwar.com: “Many individuals worldwide do view this website including individuals who are currently under investigation and Eric Garris has shown intent to disrupt FBI operations by hacking the FBI website.”

Based on that, the authors of the threat assessment recommended an investigation “be opened to determine if Eric Anthony Garris and/or Justin Raimondo are engaging in, or have engaged in, activities which constitute a threat to national security on behalf of a foreign power”.

Garris and Raimondo have known that their site was monitored by the FBI since 2011 – Raimondo gave an interview about it to Buzzfeed in August – but the initial predicate for the monitoring has been a mystery.

During the years after 9/11, the FBI monitored several nonviolent protest organizations, advocacy groups and activists, as did the Department of Homeland Security and even elements of the US military. A 2010 Justice Department inspector general’s report into the FBI monitoring found that in many cases the FBI had sufficient legal authorities for its inquiries, but criticized the bureau’s judgment for opening several initial or formal investigations of many of the groups.

“The ACLU brought this case because government surveillance of news organizations undermines democracy and interferes with journalists’ ability to do their jobs,” said Julia Harumi Mass, an attorney with the ACLU of northern California, which filed the Freedom of Information Act request for antiwar.com’s file.

It appears that the FBI’s San Francisco office considered the predicate for investigating antiwar.com to be insufficient. A 2005 document states: “Note SF declined the recommendation of Newark [redacted] Eric Garris, Justin Raimondo and the internet website www.antiwar.com.” Another heavily redacted document, from July 2004, appears to be a notification to the Newark officials from their San Francisco colleagues that the San Francisco office considered the case against antiwar.com to be meager.

The documents show that the FBI apparently monitored antiwar.com through 2008. While there is no suggestion in the documents that it continued beyond that date, the ACLU noted that the FBI withheld several documents from disclosure, as is often typical with FOIA lawsuits.

Garris said that since antiwar.com learned of the FBI surveillance in 2011, donations dried up by 20% in the following year.

“We’ve actually talked to three large contributors shortly after that who told us that they’d not feel comfortable giving us money anymore because they were afraid of the repercussions, and I don’t know how many other donors have been put off in that way,” Garris said.

The ACLU said it would continue legal action to expunge the mistaken information on antiwar.com from the FBI’s files.