Police in the university town of Boulder say they have responded to the incident at the King Soopers super market.
A law enforcement source told ABC News that officers responded to a report of someone being shot in the store’s car park, and were in turn fired upon by a suspect.
“ALERT: Active Shooter at the King Soopers on Table Mesa. AVOID THE AREA. PIO is en-route,” tweeted Boulder Police Department just before 3pm local time.
Jefferson County Sheriff’s Office says they have deployed SWAT to the scene.
A witness told FOX31 that they had seen a man in the car park dressed in tactical gear and carrying an “AR-15 style weapon.”
The King Soopers grocery store on Table Mesa is two miles south of the University of Colorado Boulder campus.
Video from the scene showed police surrounding the building as someone could be heard over the loudspeaker saying, “This is the Boulder Police Department … come out with your hands up.”
Police could then be seen removing a shirtless man with a blood covered leg from the scene in handcuffs but it is unclear if he is a suspect.
The state’s governor, Jared Polis, took to Twitter to give a statement on the incident.
“Like my fellow Coloradans, I am closely watching unfolding events at King Soopers in Boulder. My prayers are with our fellow Coloradans in this time of sadness and grief as we learn more about the extent of the tragedy,” he tweeted.
Witness Daniel Douglas said he was in the store grabbing lunch when the incident unfolded.
“As we were waiting on the pizza and we started hearing gunshots. It sounded like someone was breaking the glass at first,” Mr Douglas said.
“We started screaming and hit the ground.”
Boulder Police then took to Twitter to tell people to stay away from the “very active scene.”
“Please avoid the area of Table Mesa & Broadway! This is still a very active scene. Do NOT broadcast on social media any tactical information you might see,” they tweeted.
James Bentz, 57, told the Denver Post that he was inside the store when the shooting happened and heard a series of “pops.”
“I was then at the front of a stampede,” he told The Denver Post.
Mr Bentz added that younger people in the store helped elderly shoppers escape through a rear loading dock.
“It seemed like all of us had imagined we’d be in a situation like this at some point in our lives,” he said.
The last mass shooting at a US super market was at a Walmart in El Paso, Texas, in August 2019, in which 22 people died and 26 were injured.
Boulder’s mayor Sam Weaver thanked police and first responders for their actions.
“Words can do no justice to the tragedy that has unfolded this afternoon. Our community will soon grieve our losses, and begin our healing. Our brave police officers and first responders have the gratitude of our entire city,” he tweeted.
Boulder Community Health, a level two trauma centre, told CNN that they had received one patient from the shooting but would not release details of their condition.
The FBI say they are now on the scene to help Boulder Police Department.
“At the request of the Boulder Police Department, FBI Denver personnel are assisting with an investigation into an active shooter situation at a King Soopers grocery store on Table Mesa Drive in Boulder,” the agency tweeted.
BPD then warned residents in an area of the town to shelter in place while they “respond to report of armed, dangerous individual.”
“PD is investigating to determine if this is related to King Soopers shooting,” they tweeted.