Police on Tuesday identified a 21-year-old man as the suspect who opened fire inside a crowded Colorado supermarket in an attack that killed 10 people, including an officer, and sent terrorized shoppers and employees scrambling for cover.

Authorities said Ahmad Al Aliwi Alissa was from the Denver suburb of Arvada and that he engaged in a shootout with police Monday afternoon inside the Boulder store. The suspect was being treated at a hospital and was expected to be booked into the county jail later in the day on murder charges.

Investigators have not established a motive, but authorities believe he was the only shooter, Boulder County District Attorney Michael Dougherty said.

A law enforcement official briefed on the shooting told The Associated Press that the gunman used an AR-15 rifle, a lightweight semiautomatic rifle. Officials were trying to trace the weapon. The official was not authorized to speak publicly and spoke to AP on condition of anonymity.

The suspect鈥檚 family told investigators they believed Alissa was suffering some type of mental illness, including delusions. Relatives described times when Alissa told them people were following or chasing him, which they said may have contributed to the violence, the official told AP.

The attack was the nation鈥檚 deadliest mass shooting since a 2019 assault on a Walmart in El Paso, Texas, where a gunman killed 22 people in a rampage that police said targeted Mexicans.

Hundreds of police from throughout the Denver area responded to the attack, converging on a King Soopers supermarket in a busy shopping plaza.

SWAT officers carrying ballistic shields slowly approached the store as others escorted frightened people away from the building, which had some of its windows shattered. Customers and employees fled through a back loading dock to safety. Others took refuge in nearby shops.

鈥淭his is a tragedy and a nightmare for Boulder County,鈥� Dougherty said. 鈥淭hese were people going about their day, doing their shopping. I promise the victims and the people of the state of Colorado that we will secure justice.鈥�

Boulder Police Chief Maris Herold identified the slain officer as Eric Talley, 51, who had been with the force since 2010. He was the first to arrive after responding to a call about shots fired and someone carrying a rifle, she said.

鈥淗e was by all accounts one of the outstanding officers of the Boulder Police Department, and his life was cut too short,鈥� Dougherty said.

Dozens of police and emergency vehicles, their lights flashing, escorted an ambulance carrying the slain officer from the shooting scene after nightfall. Some residents stood along the route, their arms raised in salute.

The other dead ranged in age from 20 to 65. They were identified as Denny Strong, 20 Neven Stanisic, 23 Rikki Olds, 25 Tralona Bartkowiak, 49 Suzanne Fountain, 59 Teri Leiker, 51 Kevin Mahoney, 61 Lynn Murray, 62 and Jodi Waters, 65.

I am heartbroken to announce that my Dad, my hero, Kevin Mahoney, was killed in the King Soopers shooting in my hometown of Boulder, CO. My dad represents all things Love. I'm so thankful he could walk me down the aisle last summer.

鈥� Erika Mahoney (@MahoneyEb)

The attack in Boulder, about 25 miles (40 kilometres) northwest of Denver and home to the University of Colorado, stunned a state that has seen several mass shootings, including the 1999 Columbine High School massacre and the 2012 Aurora movie theatre shooting.

Monday鈥檚 attack was the seventh mass killing this year in the U.S., 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.

It follows a lull in mass killings during the coronavirus pandemic in 2020, which had the smallest number of such attacks in eight years, according to the database, which tracks mass killings defined as four or more dead, not including the shooter.

Rep. Joe Neguse, a Democrat whose district includes Boulder, said Tuesday on 鈥淐BS This Morning鈥� that 鈥渆nough is enough鈥� when it comes to political impasses that keep gun control laws from passing Congress.

鈥淭he time for inaction is over. It does not have to be this way. There are commonsense gun legislation reform proposals that have been debated in Congress for far too long,鈥� Neguse said. 鈥淭he gun lobby and so many others have stopped the ability to make meaningful reforms in the past, but that鈥檚 no excuse. I think the American people are tired of excuses.鈥�

Speaker Nancy Pelosi said the U.S. House stands with gun violence victims who say 鈥渆nough is enough.鈥�

鈥淎ction is needed now to prevent this scourge from continuing to ravage our communities,鈥� she said in a statement Tuesday in which she cited House passage of two background checks bills. Those bills are now pending before the Senate.

鈥淲hile we await further information on the details of this heinous crime, we continue to stand with victims, families and young people across the country saying, `Enough is enough.鈥濃��

VIDEO:

Sarah Moonshadow and her son, Nicolas Edwards, had just bought strawberries inside the supermarket when they heard gunfire. Moonshadow told The Denver Post they ducked and 鈥渏ust ran.鈥� Outside, Edwards said, arriving police pulled up next to a body in the parking lot.

鈥淚 knew we couldn鈥檛 do anything for the guy,鈥� he said. 鈥淲e had to go.鈥�

Video posted on YouTube showed one person on the floor inside the store and two more outside on the ground. What sounds like two gunshots are heard at the beginning of the video.

Investigators had just started sorting through the crime scene and conducting witness interviews, Dougherty said.

Matthew Kirsch, the acting U.S. attorney for Colorado, pledged that 鈥渢he full weight of federal law enforcement鈥� would support the investigation. He said investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the crime scene, along with FBI agents.

President Joe Biden announced that flags nationwide would be lowered in memory of the victims 鈥� an order that comes just as a previous flag-lowering proclamation expired for those killed in the Atlanta-area shootings. Together the two orders mean near-continuous national mourning for almost two weeks.

Colorado Gov. Jared Polis said in a statement that the state had seen 鈥渢he face of evil.鈥�

The King Soopers chain said in a statement that it was offering prayers and support 鈥渢o our associates, customers, and the first responders who so bravely responded to this tragic situation.鈥�

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Associated Press writers Michael Balsamo in Washington and Colleen Slevin and Jim Anderson in Denver contributed. Nieberg is a corps member for the Associated Press/Report for America Statehouse News Initiative. Report for America is a non-profit national service program that places journalists in local newsrooms to report on undercovered issues.

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Patty Nieberg And Thomas Peipert, The Associated Press

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