Police are just beginning what they say could be at least five days of investigation, and many questions about the mass shooting and the motivation behind the attack are still unclear. Here is what we know so far:
According to Boulder Police Chief Maris Herold, the first 911 calls reporting shots fired came in around 2:30 p.m. local time (4:30 p.m. ET) on Monday, and Officer Eric Talley was the first officer to arrive at the scene.
At 2:40 p.m, officers radioed that they were in a gunfight, according to audio from scanner traffic.
Scanner traffic shows police continued to report that they were being fired at with multiple rounds through at least 3:21 p.m.
“He’s armed with a rifle, our officers shot back and returned fire — we do not know where he is in the store,” one officer said.
Ambulances and multiple law enforcement agencies responded to the store, which is part of a large shopping center with a two-story strip mall next door.
At one point, police were seen moving on the roof of the store. The reason for the movements was unclear, but one witness who spoke to CNN affiliate KCNC said his relatives in the store were evacuated through the roof.
Who are the victims?
Talley, 51, was killed at the scene, police chief Herold said. Talley, a father of seven, had joined the Boulder police force in 2010, she said.
Officials did not disclose the identity of any of the other victims, saying they needed to first notify family members.
Who is the suspect?
A suspect is in custody, Herold said, but authorities did not share his identity or any information on the type of weapon used or any possible motive.
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 shooting inside the store.
One senior law enforcement source told CNN the weapon used in the shooting was an AR-15-style rifle.
Where does the investigation go next?
Police will be on the scene day and night to learn more about the violence that took place at the store, officials said.
“We will work around the clock to get this accomplished,” Herold said, adding that such a complex investigation will take at least five days to complete.
In the US so far this year, there have been at least six mass shootings with four or more killed.
CNN’s Steve Almasy, Paul P. Murphy, Melissa Gray, Keith Allen, Kelsie Smith, Deanna Hackney, Dianne Gallagher and Joe Sutton contributed to this report.