(AP) — 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’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 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.

“This is a tragedy and a nightmare for Boulder County,” Dougherty said. “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 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.

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

The death toll stunned a state that has seen several mass shootings, including the 1999 Columbine High School massacre and the 2012 Aurora movie theater shooting.

Monday’s attack was the seventh mass killing in the U.S. this year and followed last week’s March 16 shooting at three Atlanta-area massage businesses that have been described as a hate crime after a white gunman shot and killed eight people, including seven Asian American women.