A mass shooting at a Walmart store in the Texas border city of El Paso is being treated by US authorities as a domestic terrorism case.
- Dozens dead after two mass shootings in the US
- Ohio shooting: Nine killed by gunman in Dayton, Ohio
- El Paso shooting: Twenty killed after gunman opens fire in Walmart
Texas Governor Greg Abbott says Saturday's rampage appears to be a hate crime, and police cited a manifesto attributed to the suspect as evidence that the bloodshed was racially motivated.
A state prosecutor said they will seek the death penalty for suspect Patrick Crusius, 21, of Allen, Texas.
US attorney for the western district of Texas John Bash said federal authorities were treating the massacre that left 20 dead and 26 wounded 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 on Sunday.
He said the attack appeared "to be designed to intimidate a civilian population, to say the least".
Several Democratic candidates for the US presidential election denounced the rise of gun violence and repeated calls for tighter gun control measures.
At least two candidates, Mayor Pete Buttigieg of South Bend, Indiana, and El Paso native Beto O'Rourke, a former congressman, drew connections to a resurgence in white nationalism and xenophobic politics in the US.
"America is under attack from homegrown white nationalist terrorism," Buttigieg said at a candidate forum in Las Vegas.
US President Donald Trump branded the shooting "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 wrote on Twitter.
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.
El Paso Police Chief Greg Allen said the suspect was co-operating wit investigators.
"He basically didn't hold anything back," Allen said at Sunday's news conference, but declined to elaborate.
The Texas killings were followed just 13 hours later by another mass shooting in Dayton, Ohio.
There, a gunman in body armour killed nine people including his sister in less than a minute and wounded 27 others before he was shot dead by police.
Crusius comes from Allen, Texas, a Dallas suburb some 1000km east of El Paso.
A four-page statement posted on 8chan, an online message board often used by extremists, and believed to have been written by the suspect, called the Walmart attack "a response to the Hispanic invasion of Texas".
It also expressed support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand, in March.
El Paso is part of a metropolitan border area of some 2.5 million residents constituting the largest bilingual, bi-national population in North America.
Mexico's President Andres Manuel Lopez Obrador said three Mexican nationals wer among the 20 people killed in the shooting, and nine others were among 26 victims who were wounded.