At least 10 people, including a police officer, were killed by a gunman at a U.S. grocery store Monday, police said, without specifying the assailant's possible motive. 

A suspect was injured and is in custody, Boulder police commander Kerry Yamaguchi told reporters, following the incident at the King Soopers store in Boulder, a city at the eastern foot of the Rockies about 28 miles (45 kilometers) northwest of Denver in the western state of Colorado.

Yamaguchi said the suspect was believed to be the only individual with injuries who survived the bloodshed, but gave no further details. 

"We have multiple people who were killed in this incident. And I am sorry to have to report that one of them was a Boulder police officer," he told reporters. 

Eyewitnesses inside the store at the time of the shooting, which took place just before 3 p.m. local time (2100 GMT), said they heard multiple gunshots before fleeing through a back entrance. Police said they arrived minutes after the 911 call and were able to enter the building "very quickly." 

(With input from AFP, Reuters)