“This is just a tragedy that I’m having a hard time getting my arms around,” Margo told CNN.
The El Paso Police Department said the scene remained active around the Cielo Vista Mall, near the Walmart, but said there was no imminent threat to the public.
“There’s no active shooter currently, but that isn’t to say we’ve rendered the scene safe at this time,” another El Paso police spokesman, Enrique Carrillo, told reporters.
Carrillo said there were more than 20 casualties, a number that could include fatalities and people wounded.
Walmart said in a statement: “We’re in shock over the tragic events at Cielo Vista Mall… We’re praying for the victims, the community & our associates, as well as the first responders.”
Carrillo asked people who cannot reach loved ones to go to a family reunification center at a local middle school, instead of heading to the shopping center.
Multiple law enforcement agencies raced to the scene, including police, state troopers, Homeland Security agents and border patrol.
One witness said he saw at least one person inside the store with a fatal head wound, and he saw shoppers in bloodied clothes.
Other stores at the nearby mall were also locked down as police officers cleared the shopping center in the east of the city, which lies on the southern US border with Mexico.
Video posted on Twitter showed customers at one department store being evacuated with their hands up.
“Hands in the air!” an officer can be heard shouting in the footage.
Mass shootings are common in the United States. On Sunday, a teenage gunman opened fire with an assault-style rifle on the crowd at a food festival in Northern California, killing three people before fatally shooting himself.
The White House said President Donald Trump had been briefed on the shooting and was monitoring the situation.
At a Democratic presidential candidate forum in Las Vegas a clearly emotional Beto O’Rourke, a former Texas congressman who is from El Paso, broke the news to the audience that he had just heard about the deadly mass shooting in his home city.
O’Rourke said he had spoken to his wife Amy, who was driving in the city with one of their children. Addressing reporters, he teared up and struggled to deliver a short statement.
“I am incredibly saddened and it’s very hard to think about this,” he said. “El Paso is the strongest place in the world. This community is going to come together. I’m going back there right now to be with my family, to be with my home town.”