A 21-year-old man has been identified as the suspect who opened fire inside a crowded Colorado supermarket in an attack that killed 10 people, including a police officer.

Authorities said Ahmad Al Aliwi Alissa was from the Denver suburb of Arvada and that he engaged in a shootout with police on Monday afternoon inside the Boulder store.

The suspect is being treated at a hospital and is 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 gunman, 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 semi-automatic rifle and officers are trying to trace the weapon.

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

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.

Mr Dougherty said: “This is a tragedy and a nightmare for Boulder County. These 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.”

Boulder Police chief Maris Herold identified the killed 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.

“He was by all accounts one of the outstanding officers of the Boulder Police Department, and his life was cut too short,” Mr Dougherty said.

Dozens of police and emergency vehicles escorted an ambulance carrying the officer from the shooting scene after nightfall. Some residents stood along the route with 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.

The attack in Boulder, about 25 miles north-west 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’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.

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 perpetrator.