Police on Tuesday identified a 21-year-old man as the suspect who opened fire inside a crowded Colorado supermarket, and court documents showed that he purchased an assault weapon 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 Monday 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.

Authorities said Alissa was from the Denver suburb of Arvada and that he engaged in a shootout with police inside the 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 they believe he was the only shooter, Boulder County District Attorney Michael Dougherty said.

In Washington, President Joe Biden called on Congress to tighten the nation’s gun laws.

“Ten lives have been lost, and more families have been shattered by gun violence in the state of Colorado,” Biden said at the White House.

Senate Majority Leader Chuck Schumer vowed to bring forward two House-passed bills to require expanded background checks for gun buyers. Biden supports the measures, but they face a tougher route to passage in a closely divided Senate with a slim Democratic majority.

The suspect purchased the assault weapon just six days before the shooting, on March 16, according to the arrest affidavit released Tuesday. It was not immediately known where the gun was purchased.

The shooting came 10 days after a judge blocked a ban on assault rifles passed by the city of Boulder in 2018. That ordinance and another banning large-capacity magazines came after the 2018 mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, that left 17 people dead.

A lawsuit challenging the bans was filed quickly, backed by the National Rifle Association. The judge struck down the ordinance under a Colorado law that blocks cities from making their own rules about guns.

A law enforcement official briefed on the shooting said 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 said. The official was not authorized to speak publicly and spoke to AP on condition of anonymity.

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.

The gunfire sent terrorized shoppers and employees scrambling for cover. SWAT officers carrying ballistic shields slowly approached the King Soopers store while 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.

Multiple 911 calls paint a picture of a chaotic, terrifying scene, according to the affidavit.

One caller said the suspect opened fire out the window of his vehicle. Others called to say they were hiding inside the store as the gunman fired on customers. Witnesses described the shooter as having a black AR-15-style gun and wearing blue jeans and maybe body armor.

By the time he was in custody, Alissa had been struck by a bullet that passed through his leg, the affidavit said. He had removed most of his clothing and was dressed only in shorts. Inside the store, he had left the gun, a tactical vest, a semiautomatic handgun and his bloodied clothing, the affidavit said.

After the shooting, detectives went to Alissa’s home and found his sister-in-law, who told them that he had been playing around with a weapon she thought looked like a “machine gun,” about two days earlier, the document said.

A tapestry and a pillow blocked a narrow window next to the front door at the Arvada home believed to be owned by the suspect’s father. No one answered the door after several knocks, but young children occasionally pulled the pillow aside and peered out of the window. The two-story home with a three-car garage sits in a relatively new middle- and upper-class neighborhood.

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 gun, she said.

Homer Talley, 74, described his son as a devoted father who “knew the Lord.” He had seven children, ages 7 to 20.

“We know where he is,” his father told The Associated Press from his ranch in central Texas. “He loved his family more than anything. He wasn’t afraid of dying. He was afraid of putting them through it.”

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.

Leiker, Olds and Stong worked at the supermarket, said former coworker Jordan Sailas.

Olds’ grandmother choked up on the phone as she described the young woman she played a large role in raising. “She was just a very kind and loving, bubbly person who lit up the room when she walked in,” said Jeanette Olds, 71, of Lafayette, Colorado.

The attack in Boulder, about 25 miles (40 kilometers) 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 theater shooting.

Monday’s 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.

Biden announced that flags nationwide would be lowered in memory of the victims — 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.