Police said a gunman killed 10 people, including a police officer, after a standoff that lasted for hours at the King Sobers grocery market in Colorado.
The attack ended with police arresting a suspect and taking him out of the market.
Eyewitnesses broadcast the shooting incident live on YouTube.
The suspect had entered the grocery store and started shooting around 14:30 local time.
Boulder police posted a tweet about 20 minutes later, stating that there had been “shooting at King Sobers”.
Two hours later, the police urged people to avoid the area. The tweet added: “Please do not broadcast any tactical information on social media.”
However, some passers-by took pictures of the confrontation where the victims were seen near the grocery store, while the photographer shouted: “I don’t know what exactly is going on … I heard the sound of gunshots, one of them has fallen.” “There is shooting, move away.”
Colorado Governor Jared Polis said on Twitter: “Our prayers are made to the citizens of Colorado at this sad time as more about this attack is revealed.”
The White House said President Joe Biden has seen the latest news about the shooting incident.