ABC News(EL PASO, TX) — Authorities suspect a virulent anti-immigration screed posted online before the El Paso shooting was written by the alleged gunman, a top police official indicated Sunday.Asked whether the attack was a hate crime, El Paso Police Chief Greg Allen told reporters during a news conference Sunday that authorities were still working to conclusively attribute what he called the “manifesto” to the attack, but said “it’s beginning to look more solidly like that is the case.”The four-page document was reportedly posted to online message boards less than an hour before the suspect, 21-year-old Patrick Crusius, purportedly gunned down at least 20 people and injuring 26 others at an El Paso, Texas, Walmart on Saturday.The document decried what the author believed was an ongoing “invasion” of Texas by Hispanic people and what the author foresaw as the impending destruction of America. It suggested that the planned attack would give immigrants additional incentive to return to their countries of origin.Though the author indicated he did not consider himself a white supremacist, the document lambasted “race mixers” and said the U.S. should be split into territories based on race. While the document was critical of Republicans, it accused Democrats of using immigrants to engineer a nation-wide political coup.In the wake of the shooting, prominent critics of President Donald Trump accused the president of using language, like the word “invasion,” that has emboldened racists and white supremacists. The document posted online predicted the attack would be connected to Trump’s rhetoric, and the author said that his ideology predated Trump and hadn’t changed for years.On ABC’s “This Week” Sunday, Acting White House Chief of Staff Mick Mulvaney referenced the document to make a similar point, saying, “This was a sick person… no politician is to blame for that.”Other defenders of the president said Trump should not be blamed for mass shootings, which have long been a problem in the U.S. “Finger-pointing, name-calling & screaming with your keyboards is easy, yet it solves not a single problem, saves not a single life,” senior White House adviser Kellyanne Conway tweeted on Sunday.John Cohen, a former senior Department of Homeland Security official and current ABC News contributor, said he’s concerned the president’s rhetoric could still be a factor in extremist attacks in the U.S.Cohen said that while Trump’s hardline on immigration and incendiary rhetoric can mobilize and inspire his political base, “where this gets dangerous” is when it “may also serve to empower and inspire mentally unwell, violence-prone individuals who view the president’s words as giving them permission to commit acts of violence.”The document linked to the suspect appears to describe one of the weapons used in the shooting and goes into detail about the lethality of various assault rifle-style weapons and ammunition. It says that the attacker had been planning the assault for less than a month, which Cohen said underlines the challenge faced by law enforcement when it comes to discovering and stopping such plots.“It’s what the FBI and the Secret Service call ‘from flash to bang,’” Cohen said of the time between someone suddenly deciding to act and acting. “You can have somebody who’s anger is simmering for years, and then all of the sudden they hear something or something happens in their lives and they decide to act.In the document, the would-be attacker says that he would not be taken alive by police and feared having to live through a trial and receiving the death penalty.Crusius was captured by police after the shooting and a local prosecutor announced that he would file capital murder charges.John Bash, the U.S. Attorney for the Western District of Texas, said Sunday that federal authorities are investigating the incident as an act of “domestic terrorism,” meaning the suspect was allegedly intent on “coercing and intimidating a civilian population.”Bash said federal authorities are also considering bringing hate crimes and federal firearms charges against Crusius that carry the penalty of death.The mass shooting came less than 15 hours before another gunman killed nine people and wounded more than two dozen others in a popular business district in Dayton, Ohio.Copyright © 2019, ABC Radio. All rights reserved.