Police in Colorado are hunting today for a man who calmly walked into a suburban Denver Walmart and fatally shot three people before driving away.

Authorities named the suspect as Scott Ostrem, 47, and released a surveillance camera photograph of a middle-aged white man wearing a black jacket and blue jeans. They also published a photo of the red four-door hatchback he is believed to have fled in.

Ostrem "nonchalantly" entered the store in Thornton, about 16km northeast of downtown Denver, and opened fire on shoppers and employees a little after 6 pm time, Thornton Police Department spokesman Victor Avila told reporters, citing witness accounts.

"What we have determined is that it is random as of right now," Avila said. "As witnesses stated, the person came in and just shot towards a group."

Two men were killed in the shooting and a woman who was shot was taken to a hospital where she died, according to the police department.

Avila told Reuters that Ostrem was a "local" but said he could not give more details while the investigation is under way.

The Walmart was quickly surrounded by police and fire crews. About an hour after the initial alert, authorities said the threat of gunfire had ended.

Avila said there was no indication the shooting was an act of terrorism as no one has claimed responsibility.

"We can't rule anything out," he said.

Walmart customer Aaron Stephens, 44, of Thornton told Reuters he was buying groceries at a self-checkout stand when he heard gunshots and ricocheting bullets.

"The employees started screaming and the customers started screaming" as people began to flee the store, he said. "I ran out, too, because I didn't want to get shot."

Stephens said he did not see where the shooting had come from and did not see anyone struck by bullets.

NBC television affiliate 9NEWS reported a woman whose son was in the Walmart said he heard about 30 gunshots.