Officials confirmed that one injured suspect is in custody, and is being provided with medical treatment. The motive of the attack being investigated.
At least 10 people have been shot dead by a gunman at a grocery store in the western US state of Colorado, police said.
Among those dead was police officer Eric Talley, who was the first to respond to the scene at a King Soopers supermarket in Boulder, near Denver.
"We know of 10 fatalities at the scene, including one of our Boulder PD officers... Officer Tally responded to the scene – was first on the scene – and he was fatally shot," said police chief Maris Herold.
Colorado governor Jared Polis and Boulder Mayor Sam Weaver each referred to the incident as a "tragedy" in statements on Twitter.
My heart is breaking as we watch this unspeakable event unfold in our Boulder community. We are making every public safety resource available to assist the Boulder County Sheriff's Department as they work to secure the store.— Governor Jared Polis (@GovofCO) March 22, 2021
Full statement: pic.twitter.com/m8nI24pahU
"Words can do no justice to the tragedy that has unfolded this afternoon," wrote Weaver.
"Our community will soon grieve our losses, and begin our healing. Our brave police officers and first responders have the gratitude of our entire city."
Officers were reportedly shot at as they responded to a report of a person hit with gunfire in the parking area of King Soopers, a law enforcement source told ABC News.
Live-streamed video showed a white middle-aged man – shirtless and seemingly covered in blood – being detained by police and led away from the store.
The suspected gunman in the Boulder, Colorado mass shooting is a white guy. Seven reported dead!— CJ Werleman (@cjwerleman) March 22, 2021
Cue “mental issues,” “bad day,” “parking dispute,” or anything else other than what he most likely is - a white domestic terrorist. pic.twitter.com/v0ukqWtIVN
Eyewitnesses inside the store at the time of the shooting said they heard multiple gunshots before fleeing through a back entrance.
"I just nearly got killed for getting a soda and a bag of chips," Ryan Borowski, who was in the store when he heard at least eight gunshots and escaped, told CNN.
"It felt amazing that everybody was helping each other out and that our instincts were on the same page and we ran... I don't know why other people didn't, and I'm sorry that they froze. I wish that this just didn't happen."
A press conference is scheduled at 5:45 pm (2345 GMT).
Polis said he was still "closely watching unfolding events."
"My prayers are with our fellow Coloradans in this time of sadness and grief as we learn more about the extent of the tragedy," he wrote.
Armored vehicles, at least six ambulances and dozens of personnel including FBI agents and at least one SWAT team were deployed to the scene.
"Our SWAT team is on the way to assist," tweeted the sheriff's department of neighboring Jefferson County.
Unverified early images appeared to show at least three wounded people lying prone on the ground both inside and immediately outside the store, as multiple gunshots rang out.
"Active Shooter at the King Soopers on Table Mesa. AVOID THE AREA," the Boulder police department posted on Twitter.
Later, shoppers who had been led safely out of the store sat wrapped in blankets, talking to emergency workers.
The city of Boulder is located around 50 kilometers (30 miles) northwest of the state capital Denver, in the eastern foothills of the Rocky Mountains.