Ten people, including a police officer, were killed on Monday in a mass shooting at a supermarket in Boulder, Colorado.
Boulder police commander Kerry Yamaguchi said a suspect had been arrested.
He did not give more details about the shooting, or how many people were killed.
Officers escorted a shirtless man with blood running down his leg out of the store in handcuffs but authorities would not say if that was the suspect.
Boulder County District Attorney Michael Dougherty said authorities knew how many people were killed and suggested they were not releasing the number because they needed to notify families of the victims.
Mr Yamaguchi said police were still investigating and did not have details on the motive.
A witness said he heard gunshots and saw three people lying face down, two in the car park and one near the doorway.
Video posted on YouTube showed one person on the floor inside the King Soopers store and two more outside on the ground, but the extent of their injuries was not clear. What sounds like two gunshots is also heard at the beginning of the video.
One person was taken from the shooting scene to Foothills Hospital in Boulder, said Rich Sheehan, spokesman for Boulder Community Health, which operates the hospital. Mr Sheehan said he could not provide additional details but “we have been notified we will not be receiving any additional patients.”
Law enforcement vehicles and officers massed outside the store, including SWAT teams. At least three helicopters landed on the roof.
Some windows at the front of the store were broken. At one point, 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.
Sarah Moonshadow told the Denver Post that two shots rang out just after she and her son, Nicolas Edwards, finished buying strawberries. She said she told her son to get down and then “we just ran”.
Once they got outside, she said they saw a body in the car park. Mr Edwards said police were speeding into the lot and pulled up next to the body.
“I knew we couldn’t do anything for the guy,” he said. “We had to go.”
James Bentz told the Post that he was in the meat section when he heard what he thought was a misfire, then a series of pops.
“I was then at the front of a stampede,” he said.
Mr Bentz said he jumped off a loading dock at the back to escape and that younger people were helping older people to come down from it.
Colorado Governor Jared Polis tweeted a statement that his “heart is breaking as we watch this unspeakable event unfold in our Boulder community.” He called it “very much an active situation”.
Boulder police had told people to shelter in place amid a report of an “armed, dangerous individual” about five kilometres away from the grocery store.
They had said they were investigating if that report was related to the shooting at the supermarket but said at the evening news conference that it wasn’t related.
The FBI said it’s helping in the investigation at the request of Boulder police.
White House press secretary Jen Psaki tweeted that President Joe Biden had been briefed on the shooting.
Kevin Daly, owner of a restaurant about 100 metres from the supermarket, said he was in his shop when he saw police cars arriving and shoppers running from the grocery store. He said he took in several people to keep them warm. Others boarded a bus provided by Boulder police and were taken away.
The shooting comes just a month after President Joe Biden called on Congress to enact “common sense” gun law reforms. This is not the first shooting to hit the state which has been the scene of two of the most well known and deadly shootings in the US, Columbine and the Aurora theatre shootings.