Federal authorities are treating the shooting at an El Paso, Texas, Walmart that killed 20 people and wounded 26 more as a case of domestic terrorism and will pursue federal hate crime and firearm charges in connection with the massacre, officials said at a press conference Sunday.

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Patrick Wood Crusius, 21, was booked into El Paso County Jail early Sunday on capital murder charge, the El Paso Times reported. At the same Sunday morning press conference, El Paso County District Attorney Jaime Esparza said his office would seek the death penalty.

Crusius was arrested Saturday after he walked into the store around 10:40 a.m. and fired at customers and employees, police said.

He's alleged to have written a white nationalist manifesto ahead of his attack in the city near the U.S. border with Mexico.

Police said Crusius has been cooperating with authorities since his arrest. FBI officials said they have served three search warrants in the Dallas area, where the suspect is from, according to The Dallas Morning News.

A day after the El Paso shooting, a separate deadly shooting in Dayton, Ohio, killed at least nine people. At least 27 other people were injured in the early Sunday shooting.