(CNN) The horror began in the parking lot.
A gunman, wearing an armored or tactical vest, had just shot an elderly man outside the King Soopers on Monday afternoon in Boulder, Colorado. Inside the supermarket, staff watched in disbelief as the gunman stood over the man and fired again and again.
The gunman entered the store, and there were more gunshots. Employees and some shoppers fled and hid -- some to the back, some up the stairs to a room, some inside a pharmacy -- as the suspect roamed aisles, according to witnesses and an affidavit for an arrest warrant.
After shopper Ryan Borowski first heard a gunshot, it took a terrified woman running toward him for him to realize he should scatter, too.
"I turned and kept up with her, and we all ran down the aisle toward the back of the store together," where they huddled with employees. "I saw a lot of very wide eyes. I'm sure my eyes were just as terrified as everybody else's."
Calls poured in to 911 dispatchers, the first around 2:30 p.m., according to police. By 3:28 p.m., it was all over.
And what had been a regular day of errands and chores in this university town beside the Rocky Mountains northwest of Denver was shattered forever. While some shoppers and employees managed to hide, 10 people were shot dead by a lone assailant.
Monday's victims were: Boulder police Officer Eric Talley, 51; store manager Rikki Olds, 25; store employee Denny Stong, 20; store employee Teri Leiker, 51; Neven Stanisic, 23; Tralona Bartkowiak, 49; Suzanne Fountain, 59; Kevin Mahoney, 61; Lynn Murray, 62; Jody Waters, 65.
A suspect, Ahmad Al Aliwi Alissa, 21, was taken into custody after a gunfire exchange with responding police, authorities said. He is facing 10 counts of murder in the first degree, and his first court appearance is scheduled for Thursday morning.
Inside the King Soopers, pharmacy technician Maggie Montoya heard the first shot and saw everyone around her scatter, she told CNN's Anderson Cooper. She hid under a desk while a pharmacist held a chair against the door. She heard a series of gunshots and screams -- but then silence, broken only by the store music and ringing phones.
Eventually, she said, she heard the gunman give up, seemingly right outside her door, with him telling officers: "I surrender. I'm naked."
The suspect had removed all of his clothing down to his shorts and was bleeding from a "through and through" gunshot wound in his leg when he turned himself in, the affidavit reads.
Though the suspect is in custody, Colorado residents and officials are left to grapple with the violence that has impacted the community in which they felt so safe.
Boulder Mayor Sam Weaver told CNN's Pamela Brown on Wednesday he spoke with President Joe Biden about mental health, banning assault weapons and how to prevent this from happening in another community. Most importantly, Weaver said, the city and its residents will be healing for some time and that is OK.
"If you're sad and fearful to go out today, acknowledge that and take steps to take care of yourself," he said. "There will be a day when you will be able to go into a grocery store without thinking about this."
The massacre in Boulder, which comes less than a week after eight people were killed in a shooting spree at three Atlanta-area spas, has stoked fear and confusion.
"I promise that all of us here will work tirelessly ... to make sure that the killer is held absolutely and fully accountable for what he did," Boulder County District Attorney Michael Dougherty said Tuesday at a news conference.
The motive in the killings -- one of several mass shootings in the US over a one-week period -- isn't immediately known.
Investigators are also trying to determine the suspect's connection to this particular King Soopers location, according to a law enforcement official briefed on the investigation.
The choice of location was puzzling because it is about 30 minutes from his home -- and other grocery stores are closer to where he lives.
The law enforcement official said investigators are also examining possible mental health questions. Investigators believe the attack was planned, given the timing of Alissa's March 16 purchase of a Ruger AR-556 pistol. They also note there were no wounded survivors, which is uncommon in mass shootings, the official told CNN.
There was no indication that Alissa, a resident of Arvada, outside Denver, was impaired by alcohol or drugs, the affidavit reads.
It was a Ruger AR-556 pistol, modified with an arm brace, that was used in the shooting, a senior law enforcement source told CNN on condition of anonymity. The gunman was also carrying a 9 mm handgun, according to the source.
Among the items that Alissa abandoned at the store when he surrendered were two guns and a green tactical vest, the affidavit reads.
The suspect did not answer officers' questions, but he did ask to speak to his mother, according to the affidavit.
Witnesses told dispatchers they saw the assailant shoot at responding police officers, the affidavit reads. Officers exchanged gunfire with Alissa at the store, Police Chief Maris Herold said.
Eventually, officers saw that Talley -- the first Boulder police officer to arrive -- was down, shot in the head. SWAT officers removed his body, the affidavit reads.
Talley, a father of seven children ages 5 to 18, once had a different profession and "didn't have to go into policing, but he felt a higher calling," Herold said Tuesday.
"He cared about this community ... and he was willing to die to protect others," she said.
Neven Stanisic and his family left Bosnia as Serbian refugees and came to Lakewood, Colorado in search of a better life, according to his family priest, Rev. Radovan Petrovic.
"They always carry a profound memory from the old country but love it here as well," Petrovic told CNN. "To them, it's beyond comprehension that they fled war and were forced out of their homes and came here for a better life and then have this tragedy happen."
Petrovic described Stanisic as a "really good boy" who was brought up to respect other people. He loved snowboarding and basketball and would play with church members often.
Denny Stong, another King Soopers worker, was a "wise young man," according to coworker Logan Ezra Smith.
"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."
Tralona "Lonna" Bartkowiak, who managed a yoga and accessories shop in Boulder, loved to get to know her customers, friend Matisse Molina told CNN on Wednesday.
"She wanted to get to know people as who they were, like why they were in town, what their interests are. And she would rather make friends than sell (stuff)," Molina said.
Suzanne Fountain "was a person who all of her life really was about doing service, helping others," her longtime friend Helen Forster told CNN's Erin Burnett.
Forster said she met Fountain during a community theater production in the late 1980s and later hired her to work at her non-profit organization, where Fountain worked for 17 years.
"She would be the first person that people would see when they walked in the door of the non-profit building that we operate, and she just would take care of everybody. She was calm and reassuring when things were stressful," Forster said.