A gunman opened fire at a supermarket in Boulder, Colorado, on Monday, killing 10 people, including the first police officer to arrive on the scene, before the bloodied suspect was arrested in the second deadly U.S. mass shooting in a week.

Police gave few immediate details of the latest shooting and no known motive for the violence, which unfolded at about 3 p.m. at a King Soopers grocery store in Boulder, a north-central Colorado city at the eastern foot of the Rockies, about 28 miles (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.

- Reuters