US authorities say they will seek death penalty for suspect, 21-year-old Patrick Crusius of Allen, Texas.

The shooting that killed 20 people in the border city of El Paso in Texas will be handled as a case of “domestic terrorism”, US authorities have said.

The announcement came on Sunday as Mexico declared it will take legal actions to protect its citizens in the United States. Six Mexicans were among those killed in Saturday’s gun assault at a Walmart store, according to officials.

Texas Governor Greg Abbott said the attack appeared to be a hate crime, and police cited a “manifesto” they attributed to the suspect as evidence that the bloodshed was racially motivated.

A state prosecutor said they would seek the death penalty for the suspect, Patrick Crusius, 21, of Allen, Texas.

The US attorney for the western district of Texas, John Bash, said federal authorities were treating the gun assault as a case of “domestic terrorism”.

“And we’re going to do what we do to terrorists in this country, which is to deliver swift and certain justice,” Bash told reporters at a news conference.

He said the attack appeared “to be designed to intimidate a civilian population, to say the least”.

The suspect in Saturday’s attack comes from Allen, Texas, a Dallas suburb about 1,046km east of El Paso [John Locher/AP]

El Paso police chief Greg Allen said the suspect was cooperating with investigators.

“He basically didn’t hold anything back,” Allen said at Sunday’s news conference, but declined to elaborate.

Police said the suspect opened fire with a rifle on shoppers, many of them bargain-hunting for back-to-school supplies, then surrendered to officers who confronted him outside the store.

The Texas killings were followed just 13 hours later by another mass shooting in Dayton, Ohio, where a gunman in body armour killed nine people in less than a minute and wounded 27 others in the city’s downtown historic district before he was shot dead by police.

‘Xenophobic barbarism’

Crusius comes from Allen, a Dallas suburb some 1,046 km east of El Paso, which lies along the Rio Grande across the US-Mexico border from Ciudad Juarez.

A four-page statement posted on an online message board, believed to be written by the suspect before the attack, called it “a response to the Hispanic invasion of Texas”.

El Paso County is more than 80 percent Latino, according to the latest census data. Tens of thousands of Mexicans legally cross the border each day to work and shop in the city.

Mexico’s President Andres Manuel Lopez Obrador said six Mexican nationals were among the 20 people killed in the shooting, and nine others were among 26 victims who were wounded.

Marcelo Ebrard, the country’s foreign minister, said Mexico would take legal actions “in the frame of international law”.

“The president has instructed me to ensure that Mexico’s indignation translates into … efficient, prompt, expeditious and forceful legal actions for Mexico to take a role and demand that conditions are established that protect … Mexicans in the United States,” Ebrard said in a video posted on Twitter.

Jesus Seade, Mexico’s deputy foreign minister for North America, condemned the attack and called for an end to the rhetoric that incites “xenophobic barbarism”.

“The modern world can’t allow such acts of XENOPHOBIC BARBARISM, which don’t happen in a vacuum. CEASE completely the RHETORIC that incites them,” he wrote.

Trump’s rhetoric

Earlier on Twitter, US President Donald Trump referred to the shooting as “an act of cowardice”.

“I know that I stand with everyone in this country to condemn today’s hateful act. There are no reasons or excuses that will ever justify killing innocent people,” he said.

Democratic presidential candidates sought to lay blame on Trump for the El Paso shooting, saying his language against minorities promoted racial division and violence.

Cory Brooker, senator for New Jersey, said on CNN’s “State of the Union” that Trump was “responsible because he is stoking fears and hatred and bigotry”. Beto O’Rourke, a former congressman from El Paso, said he believed Trump was a white nationalist who was “encouraging more racism in this country”.

Acting White House chief of staff Mick Mulvaney rebutted the Democrats’ claims, however.

Mulvaney called recent shootings a “difficulty” facing the US that “predates this administration by many, many years”.

He said there should be some public discussion about gun laws as well as the role of social media, but stressed the shooters in El Paso and Dayton are “sick” people, and that “no politician is to blame for that”.

These were the 250th and 251st mass shootings this year in the US, according to the Gun Violence Archive, an NGO. It defines a mass shooting as an incident in which at least four people are wounded or killed in a shooting.