The incident came to an end following an hours-long standoff, with police arresting a shirtless and bloodied man who was seen being led out the King Soopers store in handcuffs.
It took place in Boulder – about 25 miles northwest of Denver and home to the University of Colorado.
Here is everything you need to know about it.
The incident began at around 2.30pm local time (8.30pm GMT) on Monday 22 March, when an attacker entered the King Soopers supermarket and began firing.
Terrorised shoppers and workers scrambled for safety as shots rang out, and SWAT officers with ballistic shields approached the store as others escorted people away from the building.
Many customers and employees fled through a back loading dock, while others took refuge in nearby shops.
"This is a tragedy and a nightmare for Boulder County,” said Boulder County District Attorney Michael Dougherty "These were people going about their day, doing their shopping. I promise the victims and the people of the state of Colorado that we will secure justice.”
What was the motive behind the shooting?
Investigators have started sorting through evidence and witness interviews, and do not yet have details on a motive for the shooting.
Matthew Kirsch, the acting US attorney for Colorado, pledged that "the full weight of federal law enforcement" will support the investigation.
He said investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the crime scene, along with FBI agents.
Who was the killed police officer?
The killed officer was identified as Eric Talley, a 51-year old who had been with Boulder police since 2010 and was one of the first officers to respond to the scene, authorities said.
"He was by all accounts one of the outstanding officers of the Boulder Police Department, and his life was cut too short," Dougherty said.
Dozens of police and emergency vehicles, their lights flashing, escorted an ambulance carrying the officer from the shooting scene after nightfall. Some residents stood along the route, their arms raised in salute.
The identities of the other nine victims were not disclosed on Monday night as police were still notifying their family members.
Is there a suspect?
Police have arrested a man, but have not revealed his name or any details.
The suspect remains in custody, and Dougherty has said the he is the only person injured in the incident that did not die, and was receiving medical care.
Dougherty said it was too early to speculate on a motive, and that the investigation involving local, state and federal agencies would take days.
What is King Soopers?
The shooting took place at a branch of King Soopers, a US supermarket brand that has chains across the Rocky Mountains area of the country and is headquartered in Denver.
The first King Soopers was opened in 1947 in Arvada, Colorado by Lloyd J. King and Charles W. Houchens, offering shoppers "friendly service, value, cleanliness, and quality."
Within a few years, King Soopers had grown to nine stores; as of March 2021, there were 117 branches.
The chain said in a statement that it was offering prayers and support “to our associates, customers, and the first responders who so bravely responded to this tragic situation.”