Boulder Police

Ten people were killed Monday, including a police officer, when a gunman opened fire at a supermarket in Boulder, Colorado, the second mass shooting in the U.S. in a week.

"These were people going about their day, doing their shopping," Boulder District Attorney  said at a late-night press briefing. He described the day's events as "a tragedy and a nightmare."

A gunman opened fire at a supermarket in Boulder, Colorado, on Monday afternoon, killing at least 10 people, including a police officer who was the first responder on the scene. This comes just days after eight women were killed in three separate shootings in Atlanta, Georgia.

A gunman opened fire at a supermarket in Boulder, Colorado, on March 22, 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.