Authorities say three people have died after a shooting at a US Walmart on Wednesday night, according to local news reports.
Multiple shots had been fired at the Grant Street Walmart in Thornton, Colorado, the Thornton Police Department confirmed in the aftermath.
No suspect is in custody at this stage.
Police spokesman Victor Avila said the man fled in a car after opening fire at the store.
"This is a very heinous act," he said.
"We don't know exactly what the motive of the person was but it was certainly a terrible act."
There was no indication the shooting was an act of terrorism.
Police investigate a shooting at a Walmart store in Thornton, Colorado. Source: AAP
They said the shooter was no longer active shortly after the attack and were yet to release any details about the circumstances of the shooting.
Police are reviewing security footage and interviewing witnesses in an attempt to establish a motive.
Witnesses told of hiding in the Walmart before being evacuated by police.
An eyewitness told 9News that he was waiting in the checkout line when he heard the gunshots.
He said he dropped to the ground and managed to flee the building as he saw employees and shoppers running for the exit.
Another woman told a reporter at the scene that her son was inside when it unfolded.
Special agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the attack.
"At this time this is NOT an active shooter. Active crime scene. We will update as info becomes available," the police department said in a tweet.
Footage from inside the Walmart shows police storm the building and question a man who was filming in the minutes after the shooting.
It was uploaded by reporter Ryan Saavedra below:
Thornton is a city of about 120,000 people roughly 16 km northeast of Denver.
The Walmart, about 16km north of downtown Denver, is in a busy shopping centre that includes restaurants, a cinema and several other stores.
Avila said that police were called to the store at about 6.30pm local time and that the gunshots had ceased by the time the first officers arrived at the scene.
Last month, a gunman in killed 58 people in the country's worst mass shooting in recent history.
- With Reuters, AFP and AAP
