EL PASO — County prosecutors will seek the death penalty against the suspected gunman in the deadly attack that took the lives of 20 people in this border city, 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.

“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 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, but Esparza said in a statement that prosecution in El Paso County will happen before a federal case.

"This horrific act was committed in our community and he should be held accountable by our community," he said.

After the 2015 mass shooting in a Charleston, S.C. church that left nine black churchgoers dead, 21-year-old white supremacist Dylann 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. He ultimately pleaded guilty and received a life sentence on the state charges.

But 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. Still, last month, U.S. Attorney General William Barr lifted an unofficial moratorium and scheduled five more for December and January.

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. El Paso police said late Sunday that all of the victims' bodies have been removed from the Walmart and taken to the medical examiner's office. Police say they won't release the victims' names until next of kin have been notified.

Jeanette Harper, and FBI special agent and spokesperson for the agency's El Paso office, said the FBI has executed three search warrants in the investigation and are conducting interviews in Dallas and San Antonio.

"Through those interviews we're putting the investigation together to be able to determine if he was part of a group or working with other individuals that were planning any future attacks," Harper said. "At this point we don't have any credible intelligence to say there is anything going on in the future."