A suspect is in custody, Boulder Police Chief Maris Herold said, but authorities have not shared any information about the type of weapon being used or a possible motive.
“We will be working around the clock to get this done,” she said, adding that such a complex investigation will take at least five days.
Herold said the murdered officer, 51-year-old Eric Talley, was one of the first officers to respond to the scene. Talley had joined the Boulder Police Department in 2010, she said.
Officials did not disclose the identities of the other victims and said they should first notify family members.
As the police investigate the chain of events, witnesses shared their experiences of the terror and panic in the store.
Steven McHugh said his son-in-law and two granddaughters were there when a gunman attacked.
“The intensity, the horror will last the rest of their lives,” he said.
Ryan Borowski told CNN he was shopping in the store when he heard the first shots, and everyone was running on the third. He said he couldn’t believe it happened in his town.
“Boulder feels like a bubble, and the bubble burst,” said Borowski. “This feels like the safest place in America, and I almost got killed for getting a soda and a bag of chips.”
He added, “It no longer feels like there is safe anywhere.”
What the authorities say has happened
Boulder Police tweeted about 2:49 PM (4:49 PM ET) that there was an “active shooter with the King Soopers on Table Mesa. AVOID THE AREA”.
In the scanner traffic, agents announced by radio that they were in a firefight. They continued to report being fired upon multiple rounds until at least 3:21 PM local time.
Talley was one of the first officers to report to the chaotic scene outside King Soopers, officials said.
“He was in all probability one of the outstanding officers of the Boulder Police Department, and his life was far too short,” said Boulder County District Attorney Michael Dougherty.
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 agents fired back and fired back – we don’t know where he is in the store,” an officer said according to a transcript of the audio.
A senior law enforcement source told CNN that the weapon used in the shooting was an AR-15 style rifle.
At one point, the police were also seen moving on the roof. The reason for the roof moves was unclear, but a witness speaking to CNN-affiliated KCNC said his relatives in the store had been evacuated through the roof.
“They hid, ran upstairs, and spent the past hour in a wardrobe,” the man said. “Half a dozen officers came in through the roof and picked them up, then told them, ‘Keep still.’ ‘
As events unfolded, KMGH’s helicopter registered a shirtless man being taken from the supermarket. The man had what looked like blood on his arm and right leg, and his hands seemed shackled behind him as two officers led him away. The man was taken away by ambulance.
It was unclear whether the man was involved in the reported active in-store shooting.
At one point, police officials tweeted that they wanted people to take shelter in another location about three miles away because of a “ report from an armed, dangerous individual. ” But it turned out that the second site was not connected to the grocery store shooting, officials said.
Calls for action against gun violence
After the three Atlanta spa shootings, the latest attack sparked calls to action and expressions of fear.
The Colorado tragedy feels particularly personal, Giffords said, given how the shooting she survived at a supermarket in Tucson devastated her community.
So far this year there have been at least six mass shootings with four or more deaths in the US.
Colorado Senator Michael Bennet also called for a national discussion on gun violence and for impartial action.
“It has been a long time since Congress has taken meaningful action to keep deadly weapons out of the wrong hands,” he said.
King Sooper is owned by the Kroger company, which said the store will remain closed during the police investigation.
CNN’s Steve Almasy, Paul P. Murphy, Melissa Gray, Keith Allen, Kelsie Smith, Deanna Hackney, Dianne Gallagher and Joe Sutton contributed to this report.