Twenty people were killed and 26 injured in a shooting at a Walmart and neighbouring stores in El Paso, Texas, on Saturday afternoon, sending shoppers racing for cover in a chaotic scene that prompted a massive police and medical response, local authorities and government officials said.

The death toll could rise, officials said, noting that some of the dozens of people injured are in critical condition.

Authorities identified a man in custody as Patrick Crusius, 21, according to two law enforcement officials familiar with the inquiry, who spoke on the condition of anonymity to discuss an ongoing investigation.

One official said that the specific number of people killed and wounded was subject to change, noting that some of the victims were critically wounded.

Authorities are also investigating a manifesto that includes remarks attacking immigrants and is sympathetic to the Christchurch attacker accused of killing 51 people in mass shootings at two New Zealand mosques this year, according to the two officials. Authorities have not confirmed that the manifesto is from the same attacker but continue to investigate it.

Local media published this image of the shooter at the Walmart.

Area hospitals said they have treated 24 victims, including one person who died at University Medical Center of El Paso, which received 13 wounded people, according to centre officials. Eleven of the victims are in critical condition, while nine are in critical but stable condition. One has left the hospital.

Local authorities said around 3 p.m. Eastern time that there were was no longer an active threat. There was conflicting information about the number of suspects. While police said one suspect was in custody, El Paso's mayor said at one point that there were three.

"I can only say that we have one suspect in custody right now," a police spokesman told reporters.

President Donald Trump tweeted that the "reports are very bad, many killed," adding that he had spoken to Texas Gov. Greg Abbott to "pledge total support of Federal Government."

A little while later, ATF's office in Dallas tweeted that it was sending agents to assist in the investigation.

City police said the shooting began about 10 a.m. local time and quickly warned that people should avoid the area of Hawkins and Gateway East boulevards. The department said in another tweet at 10:09 a.m. that the shooter remained active.

The shooting apparently began outside the Walmart.

Vanessa Saenz, a 37-year-old El Paso resident, was turning into the Walmart parking lot, with her mother and son, to buy the family's weekly groceries when she heard a few pops that sounded like fireworks.

She looked over and saw a man who seemed to be "dancing" in the air - and then she noticed a woman sprinting.

Saenz realised that the man had been shot and that these were no fireworks.

"My mom yelled, 'Just go! Speed and just go!' but of course there were people trying to dodge the bullets and running through the parking lot," Saenz said in an interview with The Post.

She also caught a glimpse of the shooter, who she said was wearing dark cargo pants, a black T-shirt, and some sort of earmuffs. He was around 5-foot-10-inches tall, thin and carrying a rifle, she said.

He was just "shooting randomly," Saenz said, and then he walked into the store and she lost sight of him.

Inside the Walmart, shoppers and employees raced to exit the store or even hide in shelves. Witnesses said Good Samaritans used their own cars to transport victims to hospitals.

The Post could not immediately reach El Paso police for more information.

CBS reported that a witness, Ray Holgin, heard at least 10 gunshots and saw at least two people with guns. He told CBS that he watched an elderly woman fall but was not sure if she had been shot.

Abbott wrote on Twitter that the state's Department of Public Safety is helping law enforcement and federal officials "to bring this tragedy to the swiftest & safest possible conclusion."

Beto O'Rourke, a former congressman from El Paso who is running for president, tweeted that news of the shooting was "heartbreaking."

"Stay safe, El Paso," he wrote.

The shooting disrupted businesses around the area.

Sofia Cervantes, 20, was going into a normal day of work at Landry's Seafood Restaurant, where she was slated to be the host. But around 11 a.m., as employees were just finishing opening duties, three Walmart employees rushed inside, she told The Post. Cervantes said the three had been working the pickup area outside of Walmart when they heard gunshots and saw people running.

They first tried to run to the Sam's Club nearby, but could not get in, she said, ending up at Landry's and staying there until a family member of one of the employees picked them up. Cervantes said she believes Landry's remains on lockdown.

Reports of a shooter also interrupted a town hall that Rep. Veronica Escobar, D-Texas, was hosting at Coronado High School, about 15 miles away from the mall.

A Facebook Live broadcast of the event showed Escobar being approached by two individuals. After a brief conversation, she told the crowd that the event would have to be cleared due to an active shooter.

She clarified that the attack was occurring at Cielo Vista rather than at the school, as people murmured.

"Sorry, sorry, sorry," she said. "I apologise. Let's take a deep breath. We've been asked by law enforcement to just send everybody home. I'm so sorry we have to end this early. Thank you all for coming. Please be safe."

Escobar joined other officials expressing their sadness over the events, tweeting that she is "utterly heartbroken" and monitoring the situation with law enforcement.

Walmart has been one of the largest gun retailers in the world, but has put new restrictions on its sales in the wake of mass shootings across the U.S.

Last year, the store announced in a statement that it was changing the minimum age required to buy a firearm or ammunition at Walmart from 18 to 21 "in light of recent events." The decision came two weeks after the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, that left 17 people dead. Walmart previously stopped selling "modern sporting rifles" such as the AR-15 in 2015 and ended handgun sales in 1993.