Court documents show a 21-year-old man identified as the suspect who opened fire inside a Colorado supermarket purchased the assault rifle less than a week before the attack that killed 10 people, including a police officer.

Supermarket employees told investigators that Ahmad Al Aliwi Alissa shot an elderly man multiple times outside the Boulder grocery store before going inside, according to the documents.

Another person was found shot in a vehicle next to a car registered to the suspect’s brother. The documents did not say where the gun was purchased.

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

The suspect’s 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 sent terrorised shoppers and employees scrambling for cover. It was the nation’s 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.

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 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 Stong, 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.