(CNN) — The 21-year-old suspect in Monday’s massacre at a Colorado supermarket — which left 10 dead including a store manager and a police officer — faces 10 counts of murder in the first degree, police said Tuesday.

Ahmad Al Aliwi Alissa, of Arvada, near Denver, is accused of opening fire Monday afternoon at the King Soopers store in the university city of Boulder, killing people ranging in age from 20 to 65, authorities said.

Police took the suspect into custody at the store Monday afternoon, less than an hour after panicked 911 callers told dispatchers of the killings unfolding there.

Alissa, who at some point was shot in the leg Monday, was booked Tuesday into county jail after being treated at a hospital, authorities said.

The motive in the Boulder killings — one of several mass shootings in the US over the last week — isn’t immediately known, and the investigation will take a long time, authorities said. Still, investigators believe he was the only perpetrator, they said.

“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 in Boulder.

Police on Tuesday also released the names of those killed: Denny Stong, 20; Neven Stanisic, 23; Rikki Olds, 25; Tralona Bartkowiak, 49; Suzanne Fountain, 59; Teri Leiker, 51; Boulder police Officer Eric Talley, 51; Kevin Mahoney, 61; Lynn Murray, 62; Jody Waters, 65.

Officers had exchanged gunfire with Alissa at the store, but it wasn’t clear who shot him, Boulder Police Chief Maris Herold said.

The suspect has “lived most of his life in the United States,” Dougherty said Tuesday, without elaborating.

The shootings in Boulder, home to the University of Colorado’s main campus nestled by the Rocky Mountains northwest of Denver, came less than a week after shootings at three spas in the Atlanta area left eight people dead.

In the last week alone, the United States has seen at least seven shootings in each of which at least four people were injured or killed.

Witnesses describe terror and panic

Witnesses have described scenes of terror and panic at the supermarket Monday.

A shooter had gunned down at least one person in the parking lot before going inside, according to Anna Haynes, a college student who was watching from her apartment window across the street.

Haynes heard what turned out to be gunshots, and then looked outside and “saw a body in the middle of the parking lot.”

“I also saw the gunman himself holding a semiautomatic rifle,” and eventually he was “shooting rapid-fire” at something before entering the building, said Haynes, editor-in-chief of the University of Colorado’s CU Independent.

“And a few seconds later, I saw people running out of the building; I heard screaming; I heard people leaving in their cars, and it just evolved into chaos within just a couple of minutes,” she said.

Ryan Borowski told CNN he was grabbing a bag of chips and a soda when he heard the first shot and saw a terrified woman running toward him. By the third shot, he was running with her toward the back of the store.

They and others gathered with employees in the back.

“I saw a lot of very wide eyes. … The employees in the back of the house didn’t know what was going on, so we told them that there was a shooter, and they told us where the exit was,” he told CNN on Tuesday.

Images from the scene — from a livestreamer and from CNN affiliate KMGH — recorded police escorting from the building a shirtless man with blood on his leg, with his hands apparently cuffed.

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

Police officer and a store manager among those killed

The slain officer, Talley, was one of the first to respond to the scene, according to Herold. Talley had joined the Boulder police force in 2010, she said.

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 Tuesday.

“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.

What authorities say happened

Police said they were called there about gunfire around 2:40 p.m. MT Monday.

Ambulances and officers from several law enforcement agencies arrived at the store, part of a large shopping center with a two-story strip mall next door.

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

The suspect was taken into custody at 3:28 p.m., Herold said.

An AR-15-style pistol, modified with an arm brace, was used in the shooting, a senior law enforcement source told CNN on condition of anonymity. A search of the suspect’s home turned up other weapons, the source said.

Calls for action against gun violence

On the heels of March 16’s three spa shootings in Atlanta, the latest attack stoked calls for action and expressions of fear.

“This past weekend it was a house party in Philadelphia. And last week it was an armed attack on Asian American women in the Atlanta area,” former US Rep. Gabrielle Giffords of Arizona, who is a shooting survivor, said in a statement. “This is not normal, and it doesn’t have to be this way. It’s beyond time for our leaders to take action.”

The tragedy in Colorado feels especially personal, Giffords said, considering how the shooting she survived outside a Tucson grocery store devastated her community.

US Sen. Michael Bennet, D-Colorado, also called for a national gun violence conversation and nonpartisan action.

“It’s long past time for Congress to take meaningful action to keep deadly weapons out of the wrong hands,” he said.

The National Rifle Association tweeted on Monday quoting the Constitution’s Second Amendment: “A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.”

King Sooper is owned by the Kroger company, which said the store will remain closed during the police investigation.

“The entire Kroger family offers our thoughts, prayers and support to our associates, customers, and the first responders who so bravely responded to this tragic situation,” the company said via its verified Twitter account.

Correction: An earlier version of this story misspelled victim Denny Stong’s last name based on information provided by the Boulder Police Department.