A suspect is in custody, Boulder Police Chief Maris Herold said, but authorities did not share any information on his identity, the type of weapon used or any possible motive.
"We will work around the clock to get this accomplished," she said, adding that such a complex investigation will take at least five days to complete.
The Boulder police tweeted about 2:49 p.m. local time — which is 4:49 p.m. ET — that there was an "Active Shooter at the King Soopers on Table Mesa. AVOID THE AREA."
In scanner traffic, officers radioed that they were in a gunfight. They continued to report that they were being fired at with multiple rounds through at least 3:21 p.m. local time.
Ambulances and multiple law enforcement agencies arrived at the store, which is part of a large shopping center with a two-story strip mall next door.
"He's armed with a rifle, our officers shot back and returned fire — we do not know where he is in the store," an officer said, according to a transcript of the audio.
Officer Eric Talley was among the first to respond to the grocery store.
CNN affiliate KMGH's helicopter recorded police leading several people away from the store.
At one point, police were also seen moving on the roof. The reason for the roof movements was unclear, but one witness who spoke to CNN affiliate KCNC said his relatives in the store were evacuated through the roof.
"They hid, ran upstairs, were hiding in a coat closet for the last hour," the man said. "Half a dozen cops came in through the roof and got them and then told them, you know, 'Stay quiet.'"
As events unfolded, KMGH's helicopter recorded a shirtless man being taken from the supermarket. The man had what appeared to be blood on his arm and right leg and his hands appeared to be cuffed behind him as two officers escorted him away. The man was taken away in an ambulance.
It was unclear whether the man was involved in the reported active shooting inside the store.