Hours later, authorities said that the mass shooting left 10 people dead, including the first police officer to arrive on the scene, and a suspect injured in the violence was arrested.

Boulder Police Chief Maris Herold said 10 people perished in the attack at King Soopers grocery. Among them were 51-year-old police officer Eric Talley, who she said was the first to respond to the shooting, about 2 miles from the University of Colorado's flagship campus.

Police said the suspected gunman, who was arrested, was believed to be the only individual with serious injuries who survived the bloodshed.

The Twitter alert from police said the incident unfolded at a King Soopers grocery store in the Table Mesa area of Boulder, a city at the eastern foot of the Rockies about 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 separate location about 3 miles away, 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 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.

He said the apparent shots sounded like they came from the checkout area at the front of the store.

Video from the scene broadcast by local television station KDVR-TV showed a shirtless, bearded man in boxer shorts being led away in handcuffs, and one person on a stretcher being loaded into an ambulance. The man in custody appeared to have a bloody leg.

Similar footage aired later by KCNC-TV showed the bloodied man in handcuffs being placed onto a stretcher and into the back of an ambulance. KCNC said no one else injured in the shooting was known to have been taken away by ambulance from the scene.

A Boulder Community Health spokesman, Rich Sheehan, said one person taken by ambulance to the hospital from the supermarket was undergoing medical treatment, but Sheehan 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. Schiller also said he saw three apparent shooting victims, but had no information about their conditions.

A short time later in a parking lot outside the store, Schiller said, he saw police officers surround a sport utility vehicle as they ordered its occupants to come out.

TV news footage showed police leading customers and workers out of the store with hands on their heads. Aerial footage also showed a heavy law enforcement presence outside the supermarket, along with at least two ambulances.

Table Mesa is home to the hilltop laboratory of the National Center for Atmospheric Research. The supermarket is located about 2 miles from the University of Colorado's flagship campus.

Colorado Governor Jared Polis said on Twitter he was monitoring events unfolding in Boulder, adding, "My prayers are with our fellow Coloradans in this time of sadness and grief as we learn more about the extent of the tragedy."

The FBI said on Twitter that its agents were assisting local police in the investigation of the supermarket shooting.

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