Police received several 911 calls, an arrest affidavit says. One caller told dispatchers someone shot out a car window and chased a man. Other callers said the shooter was wearing "an armored vest."

Grocery employees, watching through a store window, "observed the suspect shoot an elderly man in the parking lot," according to the affidavit.

"The suspect then walked up to the elderly man, stood over him and shot him multiple additional times," the affidavit reads.

One witness told police the gunman shot someone who was in a vehicle. Police eventually found someone dead in a vehicle at the parking lot, according to the affidavit. The document doesn't specify if that person and the elderly man were the same.

In scanner traffic, officers radioed that they were in a gunfight. They reported that they were being fired at through at least 3:21 p.m. local time.

Witnesses told dispatchers they saw the gunman shoot at police, the affidavit said. Officer Eric Talley was killed -- shot in the head -- and his body was removed by SWAT officers, the affidavit reads.

An officer saw the gunman walking backward toward the SWAT team to be taken into custody. The shooter "removed all of his clothing and was dressed only in shorts," according to the affidavit.

The suspect did not answer officers questions about whether there were other suspects, but he did ask to speak to his mother, the affidavit said.

Maggie Montoya, who hid under a desk in the store's pharmacy, told CNN's Anderson Cooper she heard the shooter surrender.

"I surrender. I'm naked," Montoya remembered the shooter saying.

The suspect was taken into custody at 3:28 p.m. There was no indication of alcohol or drug use, the affidavit reads.

Who are the victims?

Also killed were Denny Stong, 20; Neven Stanisic, 23; Rikki Olds, 25; Tralona Bartkowiak, 49; Suzanne Fountain, 59; Teri Leiker, 51; Kevin Mahoney, 61; Lynn Murray, 62; and Jody Waters, 65, police said Tuesday.

Talley, a father of seven children ages 5-18, once had a different profession and "didn't have to go into policing, but he felt a higher calling," Herold said.

"He cared about this community ... and he was willing to die to protect others," she said.

Olds, 25, of Lafayette, was a front-end manager at the store, her uncle, Bob Olds, told CNN.

She was a "strong, independent young woman" who was raised by her grandparents, Bob Olds said. "She was so energetic and charismatic and she was a shining light in this dark world," he told CNN.

Kroger, which owns King Soopers, said three of the victims -- Stong, Olds and Leiker -- were employees.

Olds and Stong were graduates of local high schools, according to Boulder Valley School District Superintendent Rob Anderson.

Olds was a 2013 graduate of Centaurus High School and Stong was a 2019 graduate of Fairview High School.

Fountain "was a person who all of her life really was about doing service, helping others," longtime friend Helen Forster told CNN's Erin Burnett.

Fountain had worked at a nonprofit organization and a local hospital, and later became a Medicare consultant, helping seniors.

"I think we're still a little bit in shock, and we're stunned. And I think we just have to take one day at a time and remember what she did for all of us," Forster said. "You hold someone in your heart, whether they're on the planet or not. So, I think that that's what a lot of us will be doing moving forward."

Stong was a "wise young man," coworker Logan Ezra Smith said.

"Me and him were both big Second Amendment supporters and would go shooting on the weekends," Smith said. "I will miss his smile and his laugh ... as well as his honesty. He put you in your place."

Who is the suspect?

Ahmad Al Aliwi Alissa, 21, of the Denver suburb of Arvada, was taken into custody at the shooting site, accused of killing the 10, police said.

Alissa was shot at some point in the leg. Officers had exchanged gunfire with him at the store, but it wasn't clear who shot him, Herold said.

He was treated for his injury at a hospital, but by Tuesday he had been booked into a county jail on 10 counts of murder in the first degree, police said.

Investigators don't have information about a motive. Still, they believe he was the only perpetrator, authorities said at a news conference Tuesday.

The suspect has "lived most of his life in the United States," Boulder County District Attorney Michael Dougherty said, without elaborating.

"In terms of statements that he (Alissa) may have made to investigators ... we're collecting those statements now, and we'll be providing those in the weeks ahead," Dougherty said.

A Ruger AR-556 pistol, modified with an arm brace, was used in the shooting, a senior law enforcement source told CNN on condition of anonymity. The gunman also was carrying a 9mm handgun, according to the source.

Alissa had bought a Ruger AR-556 pistol less than a week earlier, on March 16, police wrote in the affidavit, citing law enforcement databases.

A search of the suspect's home turned up other weapons, the senior law enforcement source said.

That man was the arrested suspect, Ahmad Al Aliwi Alissa, his brother, Ali Aliwi Alissa, told CNN.

Ahmad Alissa was sentenced to one year of probation and 48 hours of community service after pleading guilty to third-degree assault in 2018 for an incident that occurred in November 2017. He was accused of attacking a classmate at his Denver area high school, according to the Colorado Bureau of Investigation's database.

Where does the investigation go next?

Alissa will have his first court appearance at 8:15 a.m. MT on Thursday, according to Colorado Judicial Branch online records.

It is unclear whether Alissa will personally appear since a court document noted that "the Defendant and counsel shall appear personally unless Defendant waives, in writing, his right to appear personally."

The hearing is meant to advise Alissa of the charges he is facing, his rights and the next court date, according to a statement from the Boulder County district attorney.

Police, meanwhile, will be on the shooting 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.

A law enforcement official told CNN that the FBI continues to look at everything from the suspect's online activity to interviews with his friends and relatives.

The official said Alissa was not previously the subject of any FBI investigation and says it appears nothing in the federal system would have prohibited Alissa from buying a firearm.

Gov. Jared Polis told CNN's Pamela Brown on Tuesday that public information officers have been assigned as the point of contact for victims' families.

"I've made it known ... that when they're ready to take my call, I will of course convey the remembrance the people of Colorado have for the 10 victims," he said.

Polis said he has ordered flags to fly half-staff for 10 days to honor the 10 victims.