Though Crusius was not named by law enforcement as the shooter, local media reported his name and published what it said were CCTV images of the suspect armed with a rifle.
The Texas city’s police chief said the assault on a Walmart store on Saturday, which left another 26 people wounded, was being investigated as a potential hate crime
Police officially identified a 21-year-old white male from Allen, Texas, a Dallas suburb some 650 miles east of El Paso.
The attack came just minutes after a far-right manifesto appeared online. Senior law enforcement officials told NBC News they were “reasonably confident” the document had been posted by the shooting suspect on online message board 8chan.
If authentic, it would make it the third mass shooting this year announced in advance on the website, which often features far-right and racist content.
El Paso police chief Greg Allen said authorities were examining the manifesto, which indicated “there is a potential nexus to a hate crime”. Officials declined to elaborate and said the investigation was continuing.
The racist four-page document, titled “The Inconvenient Truth”, calls the Walmart attack “a response to the Hispanic invasion of Texas” and expresses support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand earlier this year.
He rails against automation, the “destruction of our environment” and immigration, and says his views were influenced by the so-called “Great Replacement”, a white supremacist conspiracy theory that claims people of European descent are being overwhelmed.
The 2,300-word diatribe expresses hatred of “race mixing” and suggests the US should be split up into different regions for different ethnicities.
His opinions on immigration, he writes, predate “Trump and his campaign for president”, though the author repeatedly uses talking points often wielded by the US president, including the claim “illegals” are “invading” the country, and that the mainstream media publishes “fake news”.
CNN reported the FBI has opened a domestic terror investigation into the shooting.
At least two Democratic presidential candidates, Pete Buttigieg and El Paso native Beto O’Rourke, drew connections to a resurgence in white nationalism and xenophobic politics in the US.
“America is under attack from homegrown white nationalist terrorism,” Mr Buttigieg said at a candidates forum in Las Vegas.
Mr O’Rourke partially blamed Donald Trump’s racist rhetoric, which “fundamentally changes the character of the country – and it leads to violence”.
On Twitter, Mr Trump branded the shooting “an act of cowardice”, adding, “I know that I stand with everyone in this country to condemn today’s hateful act. There are no reasons or excuses that will ever justify killing innocent people”.
The carnage ranked as the eighth-deadliest mass shooting in modern US history, after a 1984 shooting in San Ysidro, California, that claimed 21 lives.
It came just six days after the last major outbreak of US gun violence in a public place – a food festival in California where a teenager killed three people with an assault rifle and injured a dozen others before taking his own life in a hail of police gunfire.
Just hours after the El Paso attack, a gunman shot dead at least nine people outside a bar in Dayton, Ohio, early on Sunday morning.
“We are going to aggressively prosecute it both as capital murder but also as a hate crime, which is exactly what it appears to be,” Texas governor Greg Abbott told reporters of the Walmart attack, adding, “I don’t want to get ahead of the evidence”.
Refusing to call for tightening gun control measures, Mr Abbott said it was time to “focus more on memorials before we start the politics”.
In an emotional statement, El Paso county sheriff Richard Wiles railed against those “jumping in front of the cameras and offering prayers and condolences as things just keep getting worse”.
“It’s time to rise up and hold our representatives accountable at all levels. I want representatives who will stand up to racism. Who will stand up and support the diversity of our nation and our state,” he said.
Additional reporting by Reuters