BOULDER, Colo. (AP) — Law enforcement officials and former associates of a 21-year-old accused of killing 10 people at a Colorado supermarket described the suspect as someone prone to sudden rage who was suspended from high school for a sudden attack on a classmate that left the student bloodied.

Ahmad Al Aliwi Alissa, who is from the Denver suburb of Arvada, was booked into jail Tuesday on murder charges a day after the attack at a King Soopers grocery in Boulder. He was due to make a first court appearance Thursday.

Alissa had bought an assault weapon on March 16, six days before the attack, according to an arrest affidavit. Investigators have not established a motive, said Boulder County District Attorney Michael Dougherty. It was not immediately known where the suspect purchased the weapon.

Among the dead was Boulder police Officer Eric Talley, 51, who was the first to arrive after responding to a call about shots fired and someone carrying a gun, said police Chief Maris Herold.

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 The Associated Press on condition of anonymity.

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, according to an arrest affidavit.

No one answered the door Tuesday at the Arvada home believed to be owned by the suspect’s father. The two-story house with a three-car garage sits in a relatively new middle- and upper-class neighborhood.

When he was a high school senior in 2018, Alissa was found guilty of assaulting a fellow student in class after knocking him to the floor, then climbing on top of him and punching him in the head several times, according to a police affidavit.

Alissa “got up in classroom, walked over to the victim & ‘cold cocked’ him in the head,” the affidavit read. Alissa complained that the student had made fun of him and called him “racial names” weeks earlier, according to the affidavit. An Arvada police report on the incident said the victim was bloodied and vomiting after the assault. Alissa was suspended from school and sentenced to probation and community service.

One of his former high school wrestling teammates, Angel Hernandez, said Alissa got enraged after losing a match in practice once, letting out a stream of invectives and yelling he would kill everyone. Hernandez said the coach kicked Alissa off the team for the outburst.

“He was one of those guys with a short fuse,” Hernandez said. “Once he gets mad, it’s like something takes over and it’s not him. There is no stopping him at that point.”

Hernandez said Alissa also would act strangely sometimes, turning around suddenly or glancing over his shoulder. “He would say, ‘Did you see that? Did you see that?’” Hernandez recalled. “We wouldn’t see anything. We always thought he was messing with us.”

Arvada police investigated but dropped a separate criminal mischief complaint involving the suspect in 2018, said Detective David Snelling. The man also was cited for speeding in February. “Our community is obviously concerned and upset that the suspect lived here,” Snelling said.

Well after dark Tuesday night, about 100 people mourned at a makeshift memorial near the grocery that was adorned with wreaths, candles, banners reading “#Boulderstrong” and 10 crosses with blue hearts and the victims’ names. Therapy dogs were on hand to provide comfort.

Four young girls huddled in the cold, one of them crying as she reminisced about how they had protested the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida. Others recalled the 1999 massacre at Columbine High School and the 2012 Aurora movie theater massacre.

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

The other dead 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, former co-worker Jordan Sailas said.

Kim Cordova, president of the United Food and Commercial Workers Local 7, which represents more than 30 store employees, said workers did their best to get customers to safety.

“They grabbed everybody they could and they brought them to the backroom or to other areas of the store to hide or got them out through the back dock,” Cordova said. “And these poor grocery workers have just been through hell in general working through COVID this entire last year of the pandemic.”

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 AP, 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.

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

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Associated Press writers Michael Balsamo in Washington, Jim Anderson in Denver and AP staff members from around the U.S. contributed to this report. Nieberg is a corps member for the Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues.