An Instagram post connected to the 19-year-old who killed three people at a food festival in Northern California points to literature that is part of an ecosystem of white nationalist literature, according to extremism researchers.

On Sunday, police officers shot and killed Santino William Legan at the Gilroy Garlic Festival in Gilroy, California, after he opened fire at the event. Earlier the same day, a post appeared on an Instagram page attributed to him referring to a proto-fascist white supremacist manifesto by a pseudonymous 19th-century author.

NBC News has declined to identify the literature by name in order to limit its spread.

According to extremism researchers, the manifesto is part of a collection of white nationalist literature that’s been pirated and distributed for free on far-right websites. The material is part of a yearslong recruiting technique by white nationalists to target those vulnerable to their message on forums frequently populated by teenagers.

The book strongly advocates combat, violence and death along racial lines, and is filled with anti-Semitic rhetoric. It is a staple among neo-Nazis and white supremacists on extremist sites such as 4chan’s /pol/ board, National Vanguard, Stormfront and The Daily Stormer. Links to the PDF version of the book have been posted hundreds of times on 4chan in the last several years, including as recently as last month.

Daryle Lamont Jenkins, the founder and executive director of the anti-racist One People's Project organization, said this specific sort of propaganda circulation has been a strategy by white supremacists for 30 years.

“It goes back as far as the ‘80s,” Jenkins said. “I remember Aryan Nations was some of the first to start using the internet ... then the general public came on in ’95. White supremacists have been using the internet to disseminate their views for that long.”

“The people who are reading it — they’re not going to the bookstore to get Mein Kampf.”

The connection adds to other young men who have also posted racist and nationalist propaganda to social media just before perpetrating violent attacks.

Last week, FBI Director Christopher Wray told Congress that in the last nine months, a “majority of the domestic terrorism cases we’ve investigated (with a racial motive) are motivated by some version of what you might call white supremacist violence.”

The white nationalist terrorist who perpetrated the Christchurch, New Zealand, mosque shootings that killed 51 people in March posted his manifesto to 8chan, an offshoot of 4chan that also traffics in extremist white supremacy. The man who perpetrated the Poway synagogue shooting in San Diego in April was linked to a similar manifesto posted to 8chan before the shooting.

Mark Pitcavage, a senior research fellow at the Anti-Defamation League’s Center on Extremism, said texts such as the one referenced by the shooter Sunday now circulate quickly as PDFs, making it easy to get selected works into the hands of impressionable people for free.

“The invention of the PDF became a huge boon to white supremacists and people promoting white supremacy,” he said. “All sorts of other movements use it, for good and bad. They make otherwise inaccessible or expensive works available at the drop of a hat.”

Pitcavage said a free audio version of the book is even available on a prominent white nationalist blog.

“The reason they’re uploading them as PDFs is because they’re old and there are no consequences,” he said.

Users on extremist forums frequently post endorsements, excerpts or full versions of the book in PDF format alongside other fascist and white nationalist literature. One post, frequently copied and pasted over the course of several months on fringe message board 4chan, includes a link to the book referenced by the shooter next to the Unabomber’s manifesto and other common white supremacist texts.

The title of the book is often posted as a sort of motto or catchphrase indicating support for white supremacy without reference to the book. A post on the neo-Nazi site The Daily Stormer featuring the book title is littered with Nazi imagery and ends with a Nazi chant.

The text referenced by the shooter Sunday has been used specifically to recruit nihilistic, young white supremacists who don’t necessarily subscribe to traditional Nazi ideals, the Southern Poverty Law Center, a nonprofit that tracks extremist groups, has warned. The organization has twice written about the adage, which it calls a “social Darwinist philosophy that sees the survival of a pure, white race as a goal to be achieved at all costs.”