Colorado officials have identified the gunman and 10 victims aged between 20- and 65-years-old who were killed in a mass shooting inside a grocery store.
The attack at a King Soopers market in Boulder ended when police arrested the lone suspect after exchanging gunfire.
Police have not confirmed a motive. The suspect faces 10 counts of murder.
President Joe Biden has hailed a police officer and father of seven who was killed, and has called for Congress to enact stricter gun control measures.
The grocery store is located in a busy shopping plaza in Boulder, a north-central Colorado city about 30 miles (50km) away from the state capital of Denver.
It comes less than a week after a mass shooting that left eight dead, including six Asian women, at three spas in Atlanta.
President Biden on Tuesday renewed his call for a ban on assault weapons and high-capacity magazines as well as an end to loopholes in background checks.
Who are the victims?
Their names are:
- Denny Stong, 20
- Neven Stanisic, 23
- Rikki Olds, 25
- Tralona Bartkowiak, 49
- Suzanne Fountain, 59
- Teri Leiker, 51
- Eric Talley, 51
- Kevin Mahoney, 61
- Lynn Murray, 62
- Jody Waters, 65
In an emotional news conference after Monday's shooting, Police Chief Maris Herold said the crime had left her feeling "numb".
"I live three blocks up the street from that store," she said. "This is my community."
She added that her colleague who lost his life in the attack, policeman Eric Talley, was "a very kind man" with seven children, ages five to 18.
"He cared about this community. He cared about the Boulder Police Department. He cared about his family and he was willing to die to protect others," she said.
Who is the suspect?
Acting US attorney Michael Dougherty stressed the investigation was "in its early stages" but Ahmad Al Aliwi Al Issa, 21, was the only suspect involved.
He said Mr Al Issa was from Arvada, Colorado, and had lived most of his life in the US.
On Monday, authorities said the investigation would take "no less than five days to complete".
Eyewitnesses said the suspect was armed with a rifle. Unnamed police sources told US media it was an AR-15 style rifle, a semi-automatic gun that has been used in several mass shootings across the US.
Aerial footage later showed the suspected gunman handcuffed and shirtless, with an apparent injury to his leg, being put onto a stretcher for treatment.
Mr Al Issa is being held at a local hospital and is expected to be transported to the county jail later on Tuesday.
What do we know about the shooting?
It began at about 14:30 local time (20:30 GMT) on Monday when the suspect entered the supermarket and began firing.
Shoppers and employees of the store said they had to dive for cover or run to safety as the shooting unfolded.
Some of the stand-off was captured on camera by a passer-by, showing victims near the grocery store.
"I don't know what's going on... I heard gunshots, someone's down," the cameraman shouts. "There's an active shooter, get away".
Gunshots can be heard as he runs away from the shop.
The video continues, with police arriving on the scene and surrounding the market.
The Boulder police department later warned people to avoid the area and told them not to "broadcast on social media any tactical information you might see".
"We were at the checkout, and shots just started going off," said Sarah Moonshadow, who had just finished purchasing strawberries with her son, Nicholas.
She told Reuters she had tried to help one of the victims lying on the pavement outside the store, but her son pulled her away saying "we have to go".
"I couldn't help anybody," she said.
Ryan Borowski, who was also inside the store, told CNN he could not believe what had happened: "This feels like the safest spot in America, and I just nearly got killed for getting a soda and a bag of chips."
Will anything change?
The supermarket shooting was the seventh mass killing so far this year following a lull in mass killings during the pandemic last year, according to a database compiled by the Associated Press (AP), USA Today and Northeastern University. The database defines mass killings as four or more dead.
Advocates of stricter gun control have pointed to such shootings as reasons for reform.
However, the right to bear arms is protected by the Second Amendment to the US constitution and is staunchly defended by many conservatives, including ex-president Donald Trump.
The starkly divisive issue has seen little in the way of change over the years despite hundreds of mass shootings.
The state of Colorado has seen a number of mass shootings, including the 1999 Columbine High School attack that left 12 students and a teacher dead and the 2012 attack at a cinema in Aurora that killed 12 people.