A gunman killed at least 10 people, including a police officer, at a grocery store in Colorado on Monday, police said, in the latest shooting to hit a state that was the site of two of the most infamous mass murders in US history.
Witnesses at the King Soopers supermarket in Boulder County, 30 miles (50km) northwest of the state capital Denver, said they initially heard multiple loud bangs outside the shop.
Nevin Sloan, who narrowly escaped with his wife Quinlan, said panic then set in as the sound of gunshots drew closer and shoppers were forced to decide whether to stay put or make a break for the exit.
“All of a sudden we started hearing more, like ‘bang, bang, bang, bang’ and I sprinted over to her and just told her ‘hey we need to get out of here’,” he told CBS, saying they had helped others flee through an emergency exit.
“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, told CNN. “It doesn’t feel like there’s anywhere safe anymore.”
Borowski, 37, added: “It was pretty terrifying. Fastest fire drill I’ve ever been in.”
Sarah Moonshadow, 42, was at the checkout line with her adult son, Nicholas, when the gunfire began.
“And I said, ‘Nicholas, get down.’ And Nicholas ducked. And we just started listening and there, just repetitive shots ... and I just said, ‘Nicholas, run’,” she said.
Moonshadow said that she tried to attend to a victim she saw lying on the pavement outside the store, but her son pulled her away, telling her, ‘We have to go’.” 
She broke down in sobs recounting their ordeal, adding: “I couldn’t help anybody.”
Police chief Maris Herold said 10 people were killed, including officer Eric Talley, 51, who was first on the scene, paying tribute to his “heroic” colleague.
“Didn’t surprise me he was the first one there,” his father, Homer Talley, told local network KUSA, saying that his son “loved his family more than anything”.
Live-streamed video earlier showed a man – shirtless and with a bloodied leg – detained by police and led from the supermarket.
A suspect is being held in custody and was the only person suffering “serious injuries at this point”, said police commander Kerry Yamaguchi, without confirming that the man in the video was the suspect or giving any possible motive.
Dozens of armoured vehicles, ambulances and armed personnel including Federal Bureau of Investigation (FBI) agents and SWAT teams were deployed to the scene.
“Police officers’ actions fell nothing short of being heroic,” said Herold, adding the “very complex investigation” would take at least five days to complete.
“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”.
Police later identified the 21-year-old suspect as Ahmad Al Aliwi Alissa of Arvada, Colorado, who had an assault rifle, a handgun and a tactical vest, all of which were recovered at the scene of the shooting, according to an arrest affidavit filed by police.
Alissa was in stable condition after suffering a leg wound in an exchange of gunfire with responding police officers inside the store on Monday afternoon.
Authorities said that they are confident he had acted alone, though they did not offer any details on what might have motivated his actions.
“It would be premature for us to draw any conclusions at this time,” Michael Schneider, the agent in charge of the FBI’s field office in Denver, said at a news briefing.
Alissa is expected to be released from the hospital later in the day and transported to jail to await an initial court appearance on murder and other charges, officials said.
His criminal history shows a single charge for third-degree assault in 2018, according to the affidavit.
The 10 victims range in age from 20 to 65.
Eric Talley was an 11-year veteran of the Boulder police force.
Talley, 51, was the father of seven children and had recently been looking for a less dangerous job, according to a statement released by his father.
Police identified the nine other victims as Denny Stong, 20; Neven Stanisic, 23; Rikky Olds, 25; Tralona Bartkowiak, 49; Suzanne Fountain, 59; Teri Leiker, 51; Kevin Mahoney, 61; Lynn Murray, 62; and Jody Waters, 65.
The latest incident follows another mass shooting last week at Asian-owned spas in the southeastern state of Georgia that left eight dead.
Colorado Governor Jared Polis condemned the “senseless killing”.
“Today we saw the face of evil. I am grieving with my community and all Coloradans,” he said on Twitter.
“My heart aches today,” Polis said. “Flags had barely been raised back to full-mast after the tragic shooting in Atlanta that claimed eight lives, and now a tragedy here close to home at a grocery store that could be any of our neighbourhood grocery stores.”
The western state has previously suffered two of the most infamous mass shootings in US history – massacres that prompted nationwide soul-searching but did not result in major changes to the country’s lax gun ownership laws.
In 1999, two teenagers shot and killed 12 classmates and a teacher at Columbine High School.
Then in 2012, a heavily-armed man stormed a movie theatre in Aurora, murdering 12.
The gunman is now serving a life sentence without the possibility of parole.
The city of Boulder imposed a ban on “assault-style weapons” and large-capacity gun magazines in the wake of the Parkland, Florida shooting in 2018.
However, a judge last week blocked that ban, the Denver Post reported, in a decision hailed by the NRA (National Rifle Association), a pro-gun advocacy group.
President Joe Biden said last month that he wanted Congress to pass laws that would require background checks on all gun sales and ban assault weapons and high-capacity magazines.
“This administration will not wait for the next mass shooting to heed that call,” he said.
Gabrielle Giffords, a former member of Congress who survived a shooting in 2012, said it was time for action.
“This past weekend it was a house party in Philadelphia. And last week it was an armed attack on Asian American women in the Atlanta area,” she said in a statement. “This is not normal, and it doesn’t have to be this way. It’s beyond time for our leaders to take action.”
In Washington, Biden ordered US flags to fly at half-staff in honour of the victims; they had just been raised at sunset on Monday after having been lowered following the Atlanta killings.