Law enforcement agencies respond to an active shooter at a Walmart near Cielo Vista Mall in El Paso, Texas, on Saturday.

EL PASO, United States — Authorities in Texas are investigating the mass shooting at a Walmart store in El Paso on Saturday as a possible hate crime, the city's police chief said.

A 21-year-old white male suspect from Allen, a suburb of Dallas, surrendered to police outside the store after the rampage that left 20 people dead and 26 wounded.

"Right now we have a manifesto from this individual that indicates to some degree he has a nexus to potential hate crime," El Paso Police Chief Greg Allen said at a news conference.

El Paso, on the border with Mexico, has a majority Hispanic population.

"On a day that would have been a normal day for someone to leisurely go shopping turned into one of the most deadly days in the history of Texas," Governor Greg Abbott said, announcing the death toll of 20.

After speaking to the governor of Texas, President Donald Trump wrote on Twitter that the "reports are very bad, many killed" while several Democrats hoping to defeat him at next year's elections said it was past time to tackle an "epidemic" of gun violence.

It was the second fatal shooting in less than a week at a Walmart store in the US and comes after a mass shooting in California last weekend.

One suspect was taken into custody, according to police.

Footage shot on camera phones appeared to show multiple bodies lying on the ground in the store's parking lot while El Paso authorities made a desperate appeal for blood donations.

Other footage showed terrified shoppers running out of the store as gunfire echoed.

Sergeant Robert Gomez of El Paso police told reporters that Walmart was "at capacity" at the time of the shooting, with 1,000-3,000 customers inside.

Various news reports said the ages of victims being treated at hospitals ranged from two to 82 years.

A witness who gave her name as Vanessa said she had just pulled into the parking lot at Walmart when the shooting began.

"You could hear the pops, one right after another and at that point as I was turning, I saw a lady, seemed she was coming out of Walmart, headed to her car. She had her groceries in her cart and I saw her just fall," she told Fox News.

The witness told Fox that she had seen a man wearing ear muffs open fire.

"He was wearing a black t-shirt, camo colored pants. He was wearing something to cover his ears, like headphones, really thick ones.

"He was carrying a dark rifle and he was just pointing at people and just shooting and, yeah, the last thing I saw, he shot somebody that was in a corner."

After seeing the woman fall in the parking lot, "that's when I thought, okay, this is not -- these aren't fireworks ... He was just shooting randomly.

It wasn't to any particular person. It was any that would cross paths."

Individuals that were evacuated sit in a parking lot across from a Wal-Mart where a shooting occurred at Cielo Vista Mall in El Paso, Texas, on Saturday. — AFP Photos

'How you doing, brother' 

Video captured by a witness in the parking lot at Walmart in the immediate aftermath of the shooting, aired on CNN, showed three people lying motionless on the ground.

One had fallen next to a truck, while two were on the sidewalk outside the store entrance. " Ambulance! Help!" people cried as they rushed to the victims.

"How you doing, brother, how you doing," one man was heard saying on the recording.

A still captured from CCTV showed the gunman carrying what appeared to be an AK-47.

Trump, who is spending the weekend at his golf club in New Jersey, was briefed on the shooting by Attorney General Bill Barr and Texas Governor Greg Abbott, according to the White House.

It has been a particularly bad week for gun violence in the United States.

Two people died and a police officer was wounded on Tuesday at a Walmart in Mississippi.

Last Sunday a 19-year-old gunman opened fire at a food festival in northern California, killing three, including two children.

Beto O'Rourke, a former US congressman for El Paso who is now running for president, cut off his campaigning in the wake of the shooting.

"I'm incredibly saddened and it's very hard to think about this. But I tell you El Paso is the strongest place in the world, this community is going to come together," he said in a speech to supporters.

Elizabeth Warren, a senator who is among the frontrunners for the Democratic party's presidential nomination, said "far too many communities have suffered through tragedies like this already.

"We must act now to end our country's gun violence epidemic," she said.

Another presidential hopeful Cory Booker said the US had "to end this national nightmare."

"Praying for everyone affected by this unspeakable tragedy, and for our country to find the moral courage to take action to end this carnage." —  AFP