Patrick Crusius has been identified as the suspected gunman who attacked a Walmart store in El Paso, Texas, a senior law-enforcement source told The Daily Beast. Crusius’ identity was first reported by CNN.

Crusius apparently foreshadowed the attack online almost an hour beforehand, according to postings reviewed by The Daily Beast.

Crusius, 21, is a resident of Allen, Texas, outside Dallas. Police said one person is in custody for the attack. Authorities “ruled out” multiple shooters and said there are no outstanding suspects.

Twenty people were killed and scores more injured in the attack, officials said.

An eyewitness told The Daily Beast a white man in his twenties, who was dressed in black, opened fire with a rifle in front of the store’s entrance around 11 a.m. The gunman shot one person at point-blank range, the eyewitness said. Surveillance footage captured the gunman entering the store, where another eyewitness said he fired “aisle by aisle.”

El Paso Police Chief Greg Allen, who stopped short of naming the suspect, said that authorities were reviewing a “manifesto.”

“Right now we have a manifesto from this individual, that indicates to some degree, it has a nexus to potential hate crime,” Allen said at a Saturday night press conference. He went on to add, however, that authorities were still working to “validate” that it was written by the suspected shooter.

Approximately 45 minutes before the first report of gunfire, a user on the forum 8chan announced that they were planning an attack, indicated that they were in Texas, and that they would use an AK-47—similar to the weapon photographed on the gunman—to carry out the attack.

The announcement was accompanied by an anti-immigrant manifesto that invoked white supremacist terms to justify violence against Hispanic people. Authorities say three Mexican nationals were among the dead in the attack on the predominantly Hispanic city that borders Ciudad Juarez, Mexico.

The same user also uploaded a letter addressed to Crusius from Collin College, dated April 9. The college said after the shooting that Crusius had attended school there until spring 2019. The letter was first reported by Bellingcat, an investigative website.

White supremacist shooters sometimes post manifestos or links to other literature, in the hopes that a terror attack will draw attention to the writings. These manifestos are sometimes intended to inspire other acts of violence. The author of the apparent El Paso manifesto claimed to have been inspired by a manifesto written by the white supremacist who allegedly murdered 51 worshippers at a mosque in New Zealand this year. The alleged attacker of a synagogue in Poway, California, earlier this year also cited that manifesto.

Hours after the El Paso attack on Saturday, law enforcement blocked off a home in a well-kept neighborhood of brick and stucco homes with manicured lawns in Allen. At the end of a cul-de-sac where FBI and state police had gathered, ATF agents went door-to-door in an attempt to speak with neighbors.

On Sunday, a friend of Crusius’ grandparents read out a statement to reporters outside their Allen home. “We are devastated by the events of El Paso, and pray for the victims of this tragedy. Patrick Crusius is our grandson,” he read out.

“He lived with us in our house in Allen, Texas, while he attended Collin College. He moved out of our house six weeks ago, and has spent a few nights here while we were out of town.

“His driver’s license and mailing address were at our house in Allen. That connection has made us a focus of media, of course. We are talking only to law enforcement agencies, and will not be making further statements to the media. We request the media to honor our privacy.”

One neighbor, who declined to give her name, spoke highly of his grandparents. “They’re very good people, they’re very straight people,” she said, adding that she attended church with the pair.

Crusius’ father is a mental-health counselor who recently treated a gunshot victim.

—With reporting fron Dan Singer in Allen, Texas