A suspect is in custody, said Boulder police chief Maris Herold, but officials have not shared any information about the type of weapon used or any possible reason.
“We will work around the clock to achieve this,” she said, adding that such a complex investigation will take at least five days to complete.
Herold said the murdered officer, Eric Talley, 51, was one of the first policemen to respond to the scene. Talley joined the Boulder police force in 2010, she said.
The authorities did not reveal the identity of any of the other victims, saying that they should first notify their family members.
As the police investigate the chain of events, witnesses share their experience of terror and panic inside the store.
Steven McHugh said his son-in-law and two granddaughters were there when an armed man attacked him.
“The intensity, the horror will last for the rest of their lives,” he said.
Ryan Borowski told CNN that he was shopping at the store when he heard the first shots, and on the third, everyone was running. He said he couldn’t believe what happened in his city.
“Boulder looks like a bubble and the bubble has burst,” said Borowski. “This looks like the safest place in America, and I was almost killed for getting a soda and a packet of chips.”
He added: “It looks like there is no longer a safe place.”
What the authorities say happened
Boulder police tweeted around 2:49 pm (4:49 pm ET) that there was an “Active Sniper at King Soopers at Table Mesa. AVOID THE AREA.”
In the scanner traffic, the police reported that they were in a shootout. They continued to report that they were being shot several times at least until 3:21 pm local time.
Talley was one of the first officers to report the chaotic scene outside of King Soopers, officials said.
“He was, by all accounts, one of the most prominent officers in the Boulder Police Department, and his life was cut short very quickly,” said Boulder County District Attorney Michael Dougherty.
Ambulances and several public security agencies arrived at the store, which is part of a large shopping center with a two-story shopping center next door.
“He’s armed with a rifle, our policemen fired back and responded to the fire – we don’t know where he is at the store,” said an official, according to an audio transcript.
A senior police source told CNN that the weapon used in the shooting was an AR-15 rifle.
At one point, the police were also seen moving on the roof. The reason for the roof’s movements was unclear, but a witness who spoke to CNN’s affiliate, KCNC, said that his relatives at the store were evacuated from the roof.
“They hid, ran up the stairs and hid in a coat closet for the last hour,” said the man. “Half a dozen policemen came through the roof and picked them up and said to them, you know, ‘Be quiet.'”
As the events unfolded, the KMGH helicopter registered a shirtless man being taken from the supermarket. The man had what appeared to be blood on his right arm and leg and his hands felt handcuffed behind him as two policemen escorted him away. The man was taken in an ambulance.
It was not clear whether the man was involved in the alleged shooting inside the store.
At one point, the police tweeted that they wanted people in another location, about three miles away, to take shelter there because of an “account of an armed and dangerous individual”. But it turned out that the second site was not connected to the shooting at the supermarket, officials said.
Calls for action against armed violence
In the wake of the three spa shootings in Atlanta, the latest attack sparked calls for action and expressions of fear.
The Colorado tragedy seems especially personal, said Giffords, considering 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 dead in the United States.
Colorado Senator Michael Bennet also called for a national conversation about armed violence and non-partisan action.
“It is past time for Congress to take significant steps to keep deadly weapons out of the wrong hands,” he said.
King Sooper is owned by the company Kroger, 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.