Credit: Marc Piscotty/Getty Images

Police are hunting for the gunman who “nonchalantly” walked into a Walmart outside Denver on Wednesday and opened fire on customers, killing three people before fleeing the scene. Police say the killings appear to be a random act of violence.

Two men and a woman were killed in the attack that occurred at around 6 p.m. at the Walmart in Thornton, Colorado, police announced. The men died inside the store and the woman died later that night at a hospital, authorities said on Twitter.

A surveillance photo of the suspect, shared by police, showed a white man wearing a dark jacket. Although the suspect remains at large, and police have not determined a motive for the attack. Authorities have said the shooting was not an act of terrorism.

Thornton police spokesman Victor Avila said the shooter “nonchalantly” walked into the store and opened fire, the Associated Press reports. He is considered armed and dangerous.

“This is a very heinous act,” Avila said. “We don’t know exactly what the motive of the person was, but it was certainly a terrible act.”

No Walmart employees were hurt in the attack, a Walmart official told KUSA. Nearly 50 witnesses were taken to a nearby police station in the wake of the shooting before being released, according to the station. There was a brief lockdown at a nearby medical center.

Credit: Marc Piscotty/Getty Images

One shopper, Aaron Stephens, told KUSA that he was at the self-checkout when he heard the gunfire.

“At first I thought it was a firecracker,” he said. “It sounded more — it was louder … I was like, what the heck’s going on? Then I heard two more shots and I freaked out and hit the ground.”

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He added: “Employees were screaming. Customers were running like crazy. And I went out with everybody else.”

Guadalupe Perez told the AP that she heard the gunshots as she was in the store with her young son. She recalled hearing someone yell, “Let’s go. Let’s go. Leave the groceries.”

“You see all these things in the news and you go through it, it’s scary,” she told the publication. “But thank God we’re OK and nothing happened to us.”