Texas mass shooting a domestic terrorism case: US attorney

The US federal government is treating a mass shooting in Texas in which at least 20 people have been killed as a domestic terrorism case, a US attorney has said.

"We are treating this as a domestic terrorism case," John Bash, the US attorney for the Western District of Texas, said on Sunday at a media briefing, Xinhua news agency reported.

"We are conducting a methodical investigation with our partners... but with a view towards bringing federal hate  crimes charges and federal firearm charges which carry a penalty of death," said Bash.

The attack on August 3 in the border city of El Paso in Texas left 20 people dead and 26 wounded. A 21-year-old white male has been arrested.

"It's been devastating to the community," said Bash, who lived there when he was very young.

"And we're going to do what we do to terrorists in this country -- deliver swift and certain justice," Bash added.

A prayer vigil has been organised in El Paso by local community leaders for the citizens to come together at the difficult time.