Investigators are examining a screed believed to have been posted online by the suspect in Saturday's fatal shooting at a Texas shopping mall an hour before the attack, senior law enforcement officials say.

Investigators are “reasonably confident” that the suspect, identified by police as Patrick Wood Crusius, 21, of Texas, posted the diatribe on the extremist online forum 8chan before the shooting.

Terrorism experts warn that nothing should be taken at face value in the propaganda material left behind by hate crime suspects, which aim to provide fodder for social media pickup and mainstream media coverage.

The screed posted to the anonymous extremist message board railed against immigrants in Texas and pushed talking points about preserving European identity in America. The attack left at least 20 dead and 26 injured.

The document criticized both Democrats and Republicans and expressed anti-government and anti-corporate views. The author claimed to have developed those beliefs before Trump’s presidency.

The material is being considered a "nexus to a potential hate crime," El Paso police Chief Greg Allen said at a press conference.

The writing presented itself as a low-cost model for deadly attacks and envisioned the actions as part of a larger ideological war.

The document was deleted from one of 8chan's forums after the shooting began, but forum users archived the post, which contained a link to a PDF version.

The first reply to the posting was “Hello FBI.”

An initial attempt to upload the document occurred at 10:15 a.m., but was unsuccessful. The document was then uploaded a few minutes later. At 10:39 the first emergency calls reached 911.

Shoppers exit with their hands up at the nearby Cielo Vista Mall after a mass shooting at a Walmart in El Paso, Texas on Aug. 3, 2019. Jorge Salgado / Reuters

Law enforcement was already analyzing the document before the mass shooting began and had connected it to a person, but the writing didn't name a target, time, place or use the suspect's name.

The note cites the Christchurch, New Zealand, mosque shooter, an anti-immigrant white supremacist who left 51 dead in March, as an inspiration. The Christchurch shooter also linked to a livestream of his attack on Facebook, and posted that link to 8chan.

Anonymous online sites have become go-to sites for mass shooters to deposit their statements of purpose before embarking on mass murder. One month after the Christchurch shooter, a copycat who killed one person at a San Diego synagogue, also left a note on 4chan, a sister site to 8chan.

Last Saturday, a 19-year-old California man who left three dead at a Gilroy, California, garlic festival before killing himself left a note on Instagram before the attack telling users to read a specific 19th century white nationalist book.

Terrorism experts say that extremists who don’t know each other in real life are finding one another online and trying to “one up” the attacks.

“I call it ‘cascading terrorism,’” Clint Watts, a senior fellow at the Foreign Policy Research Institute, told MSNBC.

“At any time around the world there may be one or two extremists that are thinking about an attack. When they see someone in their ideology successfully conduct an attack, it inspires others,” he said.

Law enforcement sources also say it is too soon to draw any conclusion from the posting about a possible motive in Saturday's attack.

The thrust of the message feels familiar to those used in pamphlets spread by the KKK and white nationalists during the 1970s, said James Cavanaugh, a former special agent-in-charge with the Bureau of Alcohol, Tobacco, Firearms and Explosives, but given new reach in the digital age.

“These guys can reach out to any soul on the internet and poison them,” he said. "It's digital hate."

Former Deputy Attorney General Rod Rosenstein expressed a need to call out "white terrorism" in the country. "Killing random civilians to spread a political message is terrorism. FBI classifies it as domestic terrorism, but 'white terrorism' is more precise," he tweeted.

George is right. Killing random civilians to spread a political message is terrorism. FBI classifies it as domestic terrorism, but "white terrorism" is more precise. Many of the killers are lone-wolf losers indoctrinated to hate through the internet, just like Islamic terrorists. https://t.co/uyyjkoh1fR — Rod Rosenstein (@RodRosenstein) August 4, 2019

Users on the 8chan forum where the note appeared frequently write about their belief in a coming race war, plus anti-immigrant, racist and anti-Semitic conspiracy theories. White nationalist literature, slogans and images are mainstays. User posts on the message board following the incident referred to the suspect as a "martyr."

Earlier this week it was disclosed that in late May the FBI for the first time identified identity-based, anti-government and fringe political conspiracy theories as a domestic terror threat. An intelligence bulletin distributed by a local field office cited several specific criminal arrests of individuals motivated by these fringe beliefs which first grew online.