Authorities are investigating an anti-immigrant manifesto and whether it was written by the suspect in a deadly shooting in El Paso, Texas, on Saturday.

Texas officials said Saturday that police arrested a 21-year-old white man who surrendered to officers.

The manifesto describes the author's hatred towards immigrants — particularly Hispanic immigrants — and a fear they would overtake Texas and turn it into a "Democrat stronghold."

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Authorities are investigating a racist screed that may be connected to the deadly Walmart shooting in El Paso, Texas, that killed 20 people and injured 26 on Saturday.

El Paso Police Chief Greg Allen told reporters at a press conference that the attack may have had a "nexus" to a hate crime, and that investigators are working to determine whether the suspect wrote the manifesto.

Authorities have not formally identified the shooting suspect, but Allen said the suspect was a 21-year-old white male from Allen, Texas, who surrendered to the officers who approached him.

Media outlets citing unnamed law-enforcement sources have identified the suspect as 21-year-old Patrick Crusius.

Read more: 20 people killed, 26 injured injured in a mass shooting at an El Paso Walmart, Texas officials say

Law enforcement agents respond to an active shooter at a Wal-Mart near Cielo Vista Mall in El Paso, Texas, on August 3, 2019. Joel Angel Juarez/AFP/Getty Images

The manifesto, which circulated online Saturday, is an explicitly anti-immigrant text that expresses a fear that Hispanic people would overtake Texas and turn the state into a "Democrat stronghold."

The text rails against lawmakers from both parties, even slamming Republicans for their "pro-corporation" stance which the author believed could spur more immigration.

The manifesto also expressed the author's belief that his death was "likely inevitable," and predicted being gunned down by police or possibly "one of the invaders."

It also detailed his planning process for the shooting, saying he spent probably less than a month preparing. The manifesto described the type of weapons, bullets, and gear he wanted to use.