Ten people died in the shooting, including a Boulder police officer who had arrived to help those inside the store.
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 north-west of Denver.
Healthcare workers walk out of a King Sooper's Grocery store after a gunman opened fire on Monday in Boulder, Colo. It has not been confirmed if the man is the suspected gunman.
"We had multiple people who were killed in this incident, and I'm sorry to have to report that one of them was a Boulder police officer", Yamaguchi said.
Video posted on YouTube showed one person on the floor inside the store and two more outside on the ground.
Hours later, Boulder police issued a second tweet saying officers were responding to a report of an "armed, risky individual" at a nearby location, and that investigators were seeking to determine "if this is related to (the) King Soopers shooting".
Multiple law enforcement agencies have reached out to the Boulder Police Department to express their condolences and show their support for the agency.
"After calls to the police department and every local hospital and the coroner's office, we finally received a call back from the coroner's office". This is still a very active scene.
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"They're busy under the gold dome in Georgia trying to prevent people from being able to vote the same day they register", he continued. If you live nearby please stay inside for now'. Some windows at the front of the store were broken.
Authorities over a loudspeaker said the building was surrounded and that 'you need to surrender.' They said to come out with hands up and unarmed. "And we just started listening and there, just repetitive shots ... and I just said, 'Nicholas, run, '" Moonshadow later told Reuters.
He said he "couldn't tell if they were breathing".
A maskless man being escorted by officers in handcuffs.
After the bleeding man was escorted out of the store, several minutes later, a line of people who appeared to have been shopping at the time were seen being taken to safety.
Law enforcement vehicles and officers massed outside the store, including SWAT teams, and at least three helicopters landed on the roof.
Suspect Robert Aaron Long has since been charged with eight counts of murder and one count of aggravated assault. A man was arrested, maybe shooter, wasn't wearing a mask. Colorado Gov. Jared Polis called the shooting an "unspeakable event" and said he was "closely watching" the situation.