Washington: .An unidentified gunman opened fire at a Walmart store in the US state of Colorado today, killing three persons including a woman.

Hundreds of emergency responders ringed the Thornton Town Centre shopping complex for hours after the shooting was reported around 5 am IST. The Walmart is part of the centre.

"Multiple parties" were down, Thornton police said in a tweet. An hour later, the Thornton Police Department tweeted, "At this time this is not an active shooter".

The men died at the store, and the woman died at a hospital. No arrests have been made.

Aaron Stephens, 44, was in the self-checkout line at the Walmart when he heard a single shot followed by two more bursts of gunfire before people started running for the exits.

"The employees started screaming. Customers were screaming. They were running like crazy, and I ran out too because I didn't want to get killed," he said.

Investigators, who have not released any details about the circumstances of the shooting, were reviewing security footage and interviewing witnesses to get a description of the shooter.