The recordings showed phone cameras many bodies on the garage shop with other records shoppers fleeing in panic from the store showed as he heard the sound of gunfire
"Twenty innocent people from El Paso have lost their lives," Governor Greg Abbott told a news conference.
El Paso police chief Greg Allen said there were also 26 wounded, and various reports said the injured were hospitalized between two and 82 years old.
Police said between 1,000 and 3,000 shoppers were in Walmart at the time of the shooting, police confirmed that the suspect, who was arrested, was a "white man of 21" from the city of Allen, Texas.
US media said he identified himself as Patrick Cruzius and surrendered himself in to the police, near from Walmart market.
A picture taken by surveillance cameras showed the gunman carrying an AK-47 machine gun.
US President Donald Trump condemned the action as act of cowardice, that police treat as a potential hate crime.
It was the second shooting incident in less than a week at Walmart Stores in the United States and came after a shooting in California last weekend, when a suspect was arrested while police were studying an extremist statement believed to have been written by the shooter.