Three people have been killed following a shooting at a Walmart store in suburban Denver, according to police.
Two men died after a gunman opened fire inside the store in Thornton, Colorado, on Wednesday night.
A woman was also injured, but died later in hospital, according to Thornton Police Department.
On its Twitter account, the force said: "Detectives currently reviewing security footage & witnesses being interviewed for assistance with suspect(s)."
No one has been taken into custody, police said.
An hour after the initial alert was raised, officers said that the threat of gunfire had ended at the store.
Images showed police and fire crews surrounding the supermarket.
Officer Victor Avila said police were called at around 6.30pm local time and the gunshots had ended by the time emergency services arrived at the scene.
Aaron Stephens, 44, a customer at the store, said he was paying for groceries when he heard gunshots and the sound of ricocheting bullets.
"The employees started screaming and the customers started screaming. I ran out, too, because I didn't want to get shot," he said.