He said he "couldn't tell if they were breathing".
Investigators don't know yet why the suspect opened fire inside the grocery store, Dougherty said.
He said Boulder "suffered a bad and horrific mass shooting".
A woman who recently married Alissa's older brother told police that she saw him in the family home a few days earlier "playing with" a weapon she described as a "machine gun". "He cared about this community, he cared the Boulder Police Department and he cared about his family and he was willing to die to protect others".
Police in Boulder, Colorado, reported an "active shooter" at a grocery store on Monday (March 22), and aerial footage broadcast live from the scene by local media showed one person being placed in an ambulance and a bloodied man being escorted away in handcuffs.
Officers had their guns drawn, and some windows at the front of the store were broken.
Boulder Mayor Sam Weaver said that "words can do no justice to the tragedy that has unfolded this afternoon". "These were people going about their day, doing their shopping".
Monday's attack was the seventh mass killing this year in the US, following the March 16 shooting that left eight people dead at three Atlanta-area massage businesses, according to a database compiled by The Associated Press, USA Today and Northeastern University.
Calls came into the police department around 2:30 p.m. local time about shots in the area and "a possible person with a patrol rifle", Boulder police Chief Maris Herold said.
"He was by all accounts one of the outstanding officers of the Boulder Police Department, and his life was cut too short", Dougherty said.
"Officer Talley responded to the scene, was first on the scene, and he was fatally shot", Herold said.
Over the last 25 years, the Denver suburbs have seen so many high-profile shootings - in supermarkets and schools, in a movie theater and a Planned Parenthood clinic - that Colorado state legislators have passed some of the strictest gun laws in the nation.
He is expected to be released from the hospital and jailed later Tuesday, authorities said.
One video of the incident from YouTube shows two victims lying on the ground in the parking lot of the store.
Former Secretary of State and Democratic presidential nominee Hillary Clinton said in a tweet Tuesday morning that "we simply can't let this continue to be who we are".
Gov. Jared Polis said in a statement that he was grieving along with the entire state.
"I also want to stress how incredibly sorry I am for all the victims who were killed in Kings Soopers", Mr Dougherty said.
- IRS Planning To Delay April 15 Tax Deadline, Reports Say
- WWE Hall Of Famer Reacts To "Senseless Violence" Against Asian American Community
- Putin fires back at Biden's 'killer' accusation, recalls envoy
- Taiwan clears AstraZeneca vaccines, shots may start next week
- Lithuania halts the use of AstraZeneca vaccine
- China concludes espionage trial of Michael Spavor behind closed doors
- Protesters march against anti-Asian racism in US
- U.S. and China trade angry words at high-level Alaska talks
- Why Miami Beach spring break curfew could extend into April
- COVID-19 in B.C