Six people have been killed after a gunman opened fire in a deadly massacre at a supermarket car park.
In dramatic footage a bloodied man was seen being led away by officers amid reports that someone had been gunned down in a car park.
Police in Boulder, Colorado, were called to the "active shooter" incident at a grocery store on Monday.
Aerial footage broadcast live from the scene showed one person being placed in an ambulance and a bloodied man being escorted away in handcuffs.
People were filmed lying motionless in a car park after shots were fired in a supermarket at about 2.30pm local time (8.30pm GMT).
They were called to a report of someone being gunned down in the carpark, according to ABC News, only for a suspect to open fire on them.
A Twitter alert from police said the incident unfolded at a King Soopers grocery store in the Table Mesa area of Boulder, a city 28 miles northwest of Denver.
Hours later, Boulder police issued a second tweet saying officers were responding to a report of an "armed, dangerous individual" at a nearby location, and that investigators were seeking to determine "if this is related to (the) King Soopers shooting."
One man who said he was shopping inside the store with his wife told KCNC-TV in an on-camera interview that he heard three loud bangs he took for gunshots, and ran through the store with other customers to escape through a loading bay at the rear of the building.
Photographs of a man wearing only underwear with blood trailing down his leg while he is walked away by police have circulated on social media.
It is not clear if the man, who was led away to a hospital according to CBSN Denver, is a suspect.
Several minutes after he was taken away a line of people could be seen leaving surrounded by officers.
A Boulder Community Health spokesman, Rich Sheehan, said one person taken by ambulance to the hospital from the supermarket was undergoing medical treatment, but declined to describe the nature of injuries or the patient's condition.
Dean Schiller, who described himself as a journalist and who was live-streaming footage from the scene for a YouTube channel identified as ZFG Videography, said in interviews with local media he heard what sounded like gunshots as he exited the grocery store.
He also said he saw three apparent shooting victims, but had no information about their conditions
Showing the seriousness of the shooting US President Joe Biden 'has been briefed' on the incident.
His press secretary Jen Psaki said: "The President has been briefed on the shooting in Colorado and he will be kept up to date by his team as there are additional developments."
