Patrick Crusius, the alleged gunman in Saturday's mass shooting in El Paso.  Federal Bureau of Investigation

, The Texas Tribune

EL PASO, Texas — The gunman charged in the deadly attack that took the lives of 20 people in this border city has been charged with state capital murder charges, and federal authorities are separately pursuing a domestic terrorism case, law enforcement officials said Sunday.

The alleged gunman, 21-year-old Patrick Crusius of Allen, north of Dallas, is in custody after police said he opened fire at a Walmart in East-Central El Paso. He was arrested without incident and is said to be cooperating with authorities.

“I know the death penalty is something very powerful, but in this occasion it’s something that’s necessary,” El Paso District Attorney Jaime Esparza told reporters Sunday morning.

El Paso Police Chief Greg Allen said the weapon used in the shooting was purchased legally, but he did not reveal where or when it was purchased.

Crusius allegedly published a manifesto where he indicated the crime was motivated by hatred toward immigrants. El Paso police and the FBI have said they are investigating the manifesto to determine whether Crusius was the author.

John Bash, the U.S. attorney for the Western District of Texas, said the crime meets the criteria for domestic terrorism under federal law.

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“This meets [the definition], it appears to be designed to intimidate a civilian population,” he said. “And we’re going to do what we do to terrorists in this country, which is deliver swift and certain justice."

FBI Special Agent in Charge Emmerson Buie said the agency has also obtained three search warrants to execute in the Dallas area. He added that the FBI "continues to look at a number of different potential crimes” and that the FBI hate crimes fusion cell — which includes field agents, analysts and members of the agency's criminal investigations and counterterrorism divisions —has been activated.

Local authorities seeking the death penalty doesn’t mean the feds won’t do the same, however. After the 2015 mass shooting in a Charleston, S.C. church that left nine black churchgoers dead, 21-year-old white supremacistDylann Roof faced a death sentence on both state and federal charges. He was sentenced to die in federal court before the state prosecution moved forward. HeHe ultimately pleaded guilty and receiveda life sentence on the state charges.

Federal executions have been rare:The federal government has put to death three people since the death penalty was reinstated, with the last one occurring in 2003. U.S. Attorney General William Barr has scheduled five more for December and January.

It’s too early to know how those jurisdiction questions will play out in the El Paso shooting, but Texas has executed more people than any other state in the country by far — with more than 560 people put to death since capital punishment was reinstated nationally in 1976. Eleven men are scheduled to be executed before the end of the year.

Early Sunday morning, the Walmart where the shooting happened was still surrounded by police officers and yellow crime scene tape. Allen said authorities were working quickly to restore normalcy to the area.

“We’re beginning to remove the bodies from the scene,” he said.