A gunman opened fire at a supermarket in Boulder, Colorado, killing 10 people, including a police officer, before being arrested, marking the second deadly US mass shooting in less than a week.

Police gave few immediate details of the shooting and no known motive for the violence, which unfolded at about 3pm at a King Soopers grocery store in Boulder, a north-central Colorado city at the eastern foot of the Rockies, about 45 km northwest of Denver.

Frantic shoppers and employees fled for cover through the supermarket as law enforcement officers swarmed the scene located about 2 miles from the University of Colorado’s flagship campus.

Sarah Moonshadow, 42, a customer and Boulder resident who was in the store with her son, Nicholas, recounted scenes of pandemonium as gunfire rang out from inside the store.

“We were at the checkout, and shots just started going off,” Moonshadow told Reuters. “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.'”

Moonshadow said she tried to go to the aid of a victim she saw lying on the pavement just outside the store, but her son pulled her away, telling her, ‘We have to go.'”

The woman broke down in sobs, adding, “I couldn’t help anybody.”

The bloodshed came less than a week after gun violence last Tuesday that left eight people dead, including six Asian women, at three day spas in and around Atlanta. A 21-year-old man has been charged with those killings.

Boulder Police Chief Maris Herold said 10 people perished in the attack at King Soopers grocery.

Among them was 51-year-old policeman Eric Talley, an 11-year veteran of the Boulder police force, who she said was the first officer at the scene.