Police warned people in the area to stay away from the shopping center.
Reports of shots fired came shortly before 6.30 pm Mountain Time, a police spokesperson told the local media in Denver.
The Walmart is located in the Thornton Town Center which includes multiple restaurants and stores.
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It's unclear how many victims there are, the police also said.
Our partners at Denver7 are reporting that one person has died and another injured in a shooting in Thornton.
Make sure to stay with NewsCenter 16 - on-air and online - as we continue to follow this developing story.