A man accused of gunning down people at a busy Walmart in El Paso last month was indicted Thursday for capital murder, prosecutors announced.

Patrick Crusius, 21, of Allen, Texas, was indicted on one count in connection with the Aug. 3 mass shooting that left 22 dead, District Attorney Jaime Esparza said. El Paso prosecutors are seeking the death penalty for Crusius, who remains jailed without bond.

Crusius' defence lawyers did not immediately respond to requests for comment Thursday. Attorney Mark Stevens previously said he will use "every legal tool available" to prevent his client from being executed.

The El Paso County District Clerk's office said Crusius's indictment would not be publicly available until next week because it takes a few days to process and assign the case to a court.

Prosecutors have said Crusius surrendered to police after the attack saying, "I'm the shooter," and that he was targeting Mexicans. In court documents, prosecutors alleged that Crusius was the author of a screed published shortly before the shooting that said it was "in response to the Hispanic invasion of Texas."

Federal prosecutors have said they are weighing hate-crime charges against Crusius that could also carry the death penalty.

Survivors of the El Paso attack have held vigils around the city, including outside the jail in downtown El Paso where Crusius has been kept isolated from other prisoners, on suicide watch.

The massacre was the first in a series of mass shootings last month that left dozens dead and, again, brought the debate over guns into the centre of American politics.

Less than 24 hours after the shooting in Texas, a masked gunman began shooting in a nightlife district of Dayton, Ohio, killing nine people and injuring 27. These shootings were followed weeks later by another in the West Texas cities of Midland and Odessa, where authorities say one man killed seven people and wounded about two dozen others while firing from a car. The gunmen in both those attacks were killed by police.

The recent mass violence has fuelled anger among gun control and immigration advocates, and caused political blowback.

Texas Gov. Greg Abbott acknowledged weeks after the attack that "mistakes were made" when he sent a fundraising mailer encouraging supporters to "take matters into our own hands" and "DEFEND" Texas.

Abbott has pledged to invest in a domestic terrorism task force and has suggested he would support an expansion of firearm background checks. He has resisted calls to regulate the sale of military-style rifles like the one authorities say Crusius used in El Paso.

Texas Lt. Gov. Dan Patrick drew the ire of the NRA last week after expressed support for background checks.