The FBI has identified prominent conspiracy theories, including the sprawling rightwing hoax known as QAnon, as motivators for “domestic extremists” to carry out violence in the US.

The warning comes from an agency bulletin produced by the FBI’s Phoenix field office, first reported by Yahoo News. It states that “anti-government, identity based, and fringe political conspiracy theories very likely motivate some domestic extremists, wholly or in part, to engage in criminal or violent activity”.

The bulletin warns that “certain conspiracy theory narratives tacitly support or legitimize violent action”, and that “some, but not all individuals or domestic extremists who hold such beliefs will act on them”.

It further warns that conspiracy theories will continue to spread and incite violence unless social media companies make “significant efforts” to “remove, regulate, or counter potentially harmful conspiratorial content”.

Examples given in the report include the October 2018 Tree of Life synagogue massacre, which the FBI describes as being motivated by the “Zionist Occupation Government conspiracy theory”; the “Pizzagate”-inspired attack on Washington’s Comet Ping Pong restaurant in December 2016; and an incident in June 2018, when a believer in the “QAnon” conspiracy theory blocked traffic at Hoover Dam in Nevada with an armored truck.

In the latter case, the bulletin says that the man “sent letters from jail containing a distinctive QAnon slogan to President Trump”.

Elsewhere the report discusses false 2018 claims by a border militia, Veterans on Patrol, that they had discovered a child sex trafficking camp near Tucson, Arizona, and the 2012 mass shooting at Sandy Hook Elementary in Connecticut, following which “conspiracy theorists who believed the shooting was a government hoax harassed and threatened family members of the slain victims”.

Currently, several families of Sandy Hook victims are suing the conspiracy-minded broadcaster Alex Jones for what they say is his role in promoting “false flag” narratives – baseless claims that the shooting was covert operation aimed at deceiving and disarming Americans.

The bulletin accords social media a central role in promoting conspiracy theories to a wider audience. It states: “The advent of the Internet and social media has enabled promoters of conspiracy theories to produce and share greater volumes of material via online platforms that larger audiences of consumers can quickly and easily access.”

As a result, the bulletin says, “it is logical to assume that more extremist-minded individuals will be exposed to potentially harmful conspiracy theories, accept ones that are favorable to their views, and possibly carry out criminal or violent actions”.

The QAnon conspiracy theory, which has emerged in the Trump era, is based on posts by an anonymous user on the 4chan and 8chan websites that believers attempt to decipher. While the QAnon narrative is sprawling and incoherent, many believers hold that Donald Trump is leading a behind-the-scenes fight against elements of the “deep state”, including figures such as Hillary Clinton and the former FBI director James Comey.

Recently, Trump has acted in ways that have encouraged believers to think that he is sympathetic to their cause. At a Greenville rally on 17 July, Trump singled out a “beautiful baby” whose jumpsuit was adorned with the “Q” symbol. And earlier this week, Trump retweeted an account that promoted QAnon.

FBI critics, though, worry that the bulletin exhibits the same excesses as previous reports identifying threats from so-called “black identity extremists” and other groups.

Michael German is a former FBI officer and a fellow at the Brennan Center for Justice. He called the bulletin “troubling”, saying that “it continues to promote the concept of radicalization – that it’s bad ideas that put people on the path to violence”.

From the perspective of law enforcement officers, German said, “all it does is ramp up the fear of people whose ideas are strange”.

He also said the logic of the bulletin promoted a baseless fear of the internet and further justified mass surveillance: “If the spread of ideas is the problem, then preventing the spread of ideas is the solution.

“Are there conspiracy theorists who commit violence? Yes. But you’re talking about a small amount of cases alongside millions of people who believe in conspiracy theories,” German added, saying that such reports “seem to have the function of getting the FBI over the one hurdle the Justice Department has imposed” in protecting first amendment activity.

The FBI bulletin concludes with the bleak prediction that “conspiracy theories very likely will emerge, spread, and evolve in the modern information marketplace over the near term, fostering anti-government sentiment, promoting racial and religious prejudice, increasing political tensions, and occasionally driving both groups and individuals to commit criminal or violent acts”.

It does hold out hope that “significant efforts by major social media companies and websites to remove, regulate, or counter potentially harmful conspiratorial content” might change this assessment.