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 US 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 3pm at a King Soopers grocery store in Boulder, a north-central Colorado city at the eastern foot of the Rockies, about 45km northwest of Denver...