Twenty-two people were killed and a further 26 were injured on Saturday in a shooting near a shopping mall in El Paso, Texas.
This brings the number to 31 people killed during the weekend mass shootings in El Paso and Dayton, Ohio.
Patrick Crusius, from Allen, near Dallas, was charged with a single count of capital murder in the El Paso case, courts documents seen by Reuters showed, in what is likely a legal place holder to keep him in custody while an investigation is underway.
The US Attorney for the Western District of Texas John Bash said on Sunday that they would treat the case as "domestic terrorism" and do "what we do to terrorists in this country, which is deliver swift and certain justice."
The police chief said the police were analysing a "manifesto" that US media reported contained anti-immigrant and white nationalist language, which they attributed to the 21-year-old suspect as evidence that the bloodshed was racially motivated.
The four-page statement uploaded to message board 8chan called the Walmart attack "a response to the Hispanic invasion of Texas" and expressed support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand, in March.
The mass shooting prompted 8chan founder Frederick Brennan to call for the site to shut down in an interview with The New York Times. Brennan no longer has control of the site, which is now run from the Philippines by a US Army veteran, said the Times.
Governor Greg Abbott told reporters that the tragedy was "one the deadliest days in the history of Texas."
A Texas prosecutor said the state will seek the death penalty for 21-year-old Crusius if he's found guilty.
Law enforcement was first alerted of the shooting at a Walmart store near the Cielo Vista mall in El Paso, Texas, at 10:39 am local time. At about 3:30 pm local time, El Paso police department Sgt Gomez said the area was locked down and there was no longer a threat to the public.
"We're in shock over the tragic events," Walmart tweeted, adding: "we're working closely with law enforcement and will update as appropriate."
The Walmart store, which had at least 100 employees present at the time, is to remain closed for the coming days, with Chief Allen explaining that "the scene will be in play for a long period."
Watch the press conference below:
The police department tweeted that blood was "needed urgently". They said "multiple injured" were transported to various hospitals.
A hospital spokesperson for the Del Sol Medical Center in El Paso told Euronews that at least 11 people had been transferred there on Saturday.
"We will continue to utilize all available resources to respond to this senseless act of violence," the Mayor of El Paso, Dee Margo tweeted.
A family reunification centre was set up for family members, El Paso police confirmed to Euronews, and a donation page was created by the authorities "to support the victims and families of this needless violence."
Three of the fatalities and six of the wounded are Mexican nationals, according to Mexican authorities.
No law enforcement person fired a weapon, Sgt Gomez told reporters.
Police told people to avoid the Cielo Vista mall in a series of tweets on Saturday morning local time due to an active shooter. Earlier the El Paso police tweeted that there were multiple reports of "multiple shooters." Later they said there was only one shooter.
Texas' governor Greg Abbott said in a statement: "our hearts go out to the victims of this horrific shooting."
He said the state had deployed troopers, special agents, tactical teams and aircraft.
People were stranded as police secured the area, according to local US news reports. El Paso is a city of more than 600,000 people located in the US' southern border with Mexico.
Crusius grandparents said they were devastated by the attack.
"He lived with us in our house in Allen, Texas, while he attended Collin College," the statement said, read aloud by a family friend to reporters outside the home on Sunday. "He moved out of our house six weeks ago, and has spent a few nights here while we were out of town."
It's not clear when a court appearance will take place.
US President Donald Trump said he had spoken with the state's governor "to pledge total support of Federal Government" and described it as "an act of cowardice."
"There are no reasons or excuses that will ever justify killing innocent people," he added.
Mexican President Andres Manuel Lopez Obrador offered his "condolences to the families of the victims, both American and Mexican."
A few hours later, a shooting in Dayton, Ohio, resulted in the death of nine people. the two incidents come less a week after a shooting at a garlic festival in Northern California killed three people and injured 15.