Alleged El Paso gunman Patrick Crusius indicted for capital murder by grand jury

Prosecutors plan to seek the death penalty.

Patrick Crusius, the alleged gunman in the El Paso shooting, has been indicted for capital murder by a grand jury in Texas.

Crusius, 21, allegedly cased the Walmart -- located near the U.S.-Mexico border -- to size up the clientele prior to carrying out the attack that killed 22 people and injured dozens more on Aug. 3, according to authorities. He allegedly told investigators following his arrest that he set out to kill as many Mexicans as he could.

Crusius also allegedly wrote a "manifesto" that discussed his hate for immigrants and Mexicans prior to the massacre.

El Paso County District Attorney Jaime Esparza announced the capital murder charge, the "highest" in the state of Texas, in a press release on Thursday. Esparza will seek the death penalty, according to the release.

Weeks before the shooting, Crusius' mother contacted police to express concern over her son owning an assault rifle due to his age, maturity level and lack of experience, the family's attorneys, Chris Ayres and R. Jack Ayres, told ABC News in August.

The Crusius family lives in Allen, Texas, a suburb of Dallas, about 650 miles east of El Paso.

Crusius was placed on suicide watch at the El Paso County Detention Facility on Aug. 19, ABC El Paso affiliate KVIA reported. Previously, he was being held in a single 7-by-11-foot cell due to the "danger" he faced from the general population, according to the local station.

ABC News could not immediately reach an attorney for Crusius.