Authorities used security video to help identify him.
Two men and one woman are dead after a shooting at a Walmart in Thornton, Colo.
Avila said there was no indication the shooting was an act of terrorism as no one has claimed responsibility.
"This is a very heinous act", Avila said, adding that the motive was unknown.
"From the witness accounts, he nonchalantly walked in to the front of the business through the south door, and just raised the firearm and began firing", Thornton said. "We don't know exactly what the motive of the person was, but it was certainly a awful act". The woman died later at a hospital.
No information has been released by police about possible injuries or fatalities.
Aaron Stephens, 44, was in the self-checkout line at Walmart when he heard a single shot fired then more bursts of gunfire.
"The employees started screaming and the customers started screaming" as people began to flee the store, he said. "Customers were screaming. They were running like insane, and I ran out too because I didn't want to get killed", he said.
Thornton police reported "multiple parties down" and advised people to stay away from the area as dozens of police cruisers and emergency vehicles raced to the scene. "Let's go. Leave the groceries". "But thank God we're OK and nothing happened to us".
A Walmart spokesman, Ragan Dickens, said the company was working with investigators.