Within hours of the massacre Saturday at an El Paso Walmart, law enforcement officials confirmed they were investigating an anti-immigrant screed that the suspect, a 21-year-old with North Texas ties, appeared to post online before the shooting.

Patrick Crusius graduated in 2017 from Plano Senior High School. He's shown here in 2016 (Plano ISD)

The "manifesto," which is heavy on racist tropes, states "political" and "economic" reasons, among others, for an unspecified planned attack.

Authorities hadn't confirmed a possible motive in the shooting, but law enforcement officials say they believe Patrick Crusius was the gunman who killed at least 20 people Saturday and wounded more than two dozen others.

Attempts to reach his family members were unsuccessful late Saturday.

Authorities believe the suspect wrote the unsigned document, which appears to have been first posted on online forum 8chan.

The author states the attack is a response to a "Hispanic invasion" and says immigration is "detrimental to the future of America."

Three Mexican citizens were among those killed, Mexico's president Andrés Manuel López Obrador said in a video on Twitter.

The author also wrote that he supports the Christchurch shooter, referring to the man charged with 51 counts of murder and 40 counts of attempted murder after authorities say he carried out attacks on two mosques earlier this year in New Zealand.

The document also outlines what weapons the shooter planned to use in the attack — a WASR-10, which he describes as a "civilian version" of an AK-47, and possibly an AR-15.

"I didn't spend much time at all preparing for this attack. Maybe a month, probably less," he writes in the four-page document. "I have to do this before I lose my nerve."

In the last line of the manifesto, the shooter echoes white supremacist rhetoric, saying he's honored to fight to "reclaim my country."

This CCTV image obtained by KTSM 9 news channel shows the gunman identified as Patrick Crusius, 21 years old, as he enters the Cielo Vista Walmart in El Paso on Aug. 3, 2019. The gunman, armed with an assault rifle, opened fire on shoppers at a packed Walmart store, reportedly killing at least 20 people in the latest mass shooting in the United States. (COURTESY OF KTSM 9 / Getty Images)

Public records show Crusius and his family have lived in Allen.

A Plano ISD spokeswoman confirmed that a student named Patrick Crusius graduated from Plano Senior High School in 2017. And public records showed he attended Collin College in Plano.

A statement from the community college said a Patrick Crusius was enrolled from fall 2017 through spring 2019. The statement said Collin College officials are "prepared to cooperate fully with state and federal authorities in their investigation of this senseless tragedy."