A gunman killed multiple people including a police officer at a grocery store in Colorado on Monday, police said, in the latest shooting to hit the western state – scene of two of the most infamous US mass murders.
One "person of interest" is injured and in custody, police commander Kerry Yamaguchi told reporters, following the incident at the King Soopers store in Boulder, some 30 miles (50 kilometres) northwest of the state capital Denver.
"We have multiple people who were killed in this incident. And I am sorry to have to report that one of them was a Boulder police officer," said Yamaguchi, without confirming the number of deaths or any possible motive.
Multiple US media reports said at least six people were killed in the shooting, citing anonymous law enforcement sources.
Live-streamed video earlier had showed a white middle-aged man — shirtless and seemingly covered in blood — detained by police and led from the supermarket.
The suspect in custody was the only person suffering "serious injuries at this point," Yamaguchi said, without confirming that the man in the video was that suspect.
President Joe Biden — who last month called on Congress to enact "commonsense" gun law reforms — has been briefed on the shooting, White House Press Secretary Jen Psaki tweeted.
"My heart is breaking as we watch this unspeakable event unfold in our Boulder community," said Colorado governor Jared Polis, calling the incident a "senseless tragedy."
"Words can do no justice to the tragedy that has unfolded this afternoon," tweeted Boulder Mayor Sam Weaver.
"Our community will soon grieve our losses, and begin our healing."
Eyewitnesses inside the supermarket 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."
Unverified early images livestreamed by an eyewitness showed at least three people lying prone on the ground both inside and immediately outside the store, before multiple new gunshots rang out.
Dozens of armoured vehicles, ambulances and armed personnel including FBI agents and SWAT teams were deployed to the scene.
Later, shoppers who had been led safely out of the store sat wrapped in blankets, talking to emergency workers in the snow-covered parking area.
"Our hearts are broken over this senseless act of violence," said King Soopers spokeswoman Kelli McGannon, praising "first responders who so bravely responded to these acts of violence."
The shooting follows another mass shooting last week at Asian-owned spas in the southeastern state of Georgia that left eight dead.
Colorado has previously suffered two of the most infamous mass shootings in US history.
In 1999, two teenage boys shot and killed 12 classmates and a teacher at Columbine High School before dying by suicide.
Then in 2012, a heavily armed man stormed a movie theatre showing a Batman film in Aurora, Colorado, murdering 12. The gunman is now serving a life sentence without the possibility of parole.