ARVADA, Colo. — Maybe he didn't quite fit in at his midsize high school in this Denver suburb, but Ahmad Al Aliwi Alissa certainly didn't stand out. One of 11 siblings in a family that emigrated from Raqqa, Syria, two decades ago, Alissa seemed to get along with others in his early teens — a moon-faced boy who wrestled but perhaps wanted for friends.

“He was a pretty chill kid from what I can remember,” said Mark Dorokhov, who said he often ate lunch with Alissa during the short period that Dorokhov attended Arvada West High School. “He wasn’t like a popular kid or anything. And he wasn’t like the high school loser, either. He was just kind of in-between. He was like me, I guess.”

That mild persona soon unraveled. In November 2017, his senior year, the man accused of killing 10 people in a Boulder grocery store this week stood up in class and assaulted an unsuspecting student, pummeling him in the head and face for an alleged ethnic slur. He pleaded guilty to misdemeanor assault and was sentenced to probation and community service.

About the same time, Alissa threatened his wrestling teammates after losing a match. “He got super mad and started throwing his head gear. He was saying, ‘I’m going to kill you guys’ and walked out,” Angel Hernandez said. His teammates were stunned.

“We were kind of quiet about it. We kind of went ahead and went along with practice. Coach was like, ‘What the heck just happened?’” Hernandez said.

Alissa never returned to the team.

Less than three years later, a month shy of his 22nd birthday, the anonymous high school wrestler became the short, shirtless, bearded suspect in the second mass shooting in the United States in less than a week.

Boulder residents paid their respects to the lives lost on March 22 and reflected on the long history of high-profile mass shootings in Colorado. (Alexander Rosen, Lance Murphey, Whitney Shefte, Whitney Leaming/The Washington Post)

“Are they seriously saying he killed 10 people? This doesn’t make sense,” said a cousin in Syria who spoke on the condition that he be identified by only his first name, Abdullah. “How can this be true?

“Ahmad’s whole family are good people. They never had problems, not in Syria or in the U.S.”

The Alissa family appeared to prosper after arriving in the United States, opening and acquiring several restaurants that served Middle Eastern food, according to a video on a local news channel.

The family lives in a suburban neighborhood in this town of more than 100,000 people just northwest of Denver, with well-kept homes and upscale cars in the driveways. According to the affidavit released by police, Alissa lives on the top floor of a large two-story house. On Tuesday, the occupants covered the windows and occasionally peeked out at the media gathered on the street outside. No one answered the door.

Steve Weber, a neighbor two houses down, said the suspect’s family moved in about a year and a half ago and has many visitors but little interaction with the people nearby. He described the community as crime-free.

The police affidavit said that Alissa bought a gun on March 16 — the same day the Georgia shooting left eight people dead — that another resident of the home described as a “machine gun.”

The document said the weapon was a Ruger AR-556 pistol.

“Alissa had been talking about having a bullet stuck in the gun and was playing with the gun,” according to the affidavit. Others in the home were “upset with Alissa for playing with the gun in the house and took the gun,” according to the woman cited by police in the affidavit. She told police she believed Alissa had retrieved the weapon.

A Facebook profile that appears to be Alissa’s contains posts about martial arts and Islam, with no evidence of “any radical or extremist views,” according to an analysis Tuesday by the SITE Intelligence Group, which monitors online extremism. Analysts there reviewed an archived version of the Facebook page, which has been removed from the platform.

The profile shows that Alissa “was born in Syria and came to the U.S. as a toddler in 2002. He studied computer science and expressed an interest in wrestling and mixed martial arts (MMA). Alissa also frequently discussed PlayStation 4, Islam, and his stance against same-sex marriage.”

“We still don’t know what his motive was, or if he had one at all. But what I can say is that based on what I’ve seen of his social media presence, he didn’t even remotely suggest having radical Islamist leanings, or really radical leanings of any kind,” said Rita Katz, executive director of SITE. “There are already some suggesting he was a jihadi or anti-Trump terrorist, but social media posts they cite as evidence don’t really back it up.”

Alissa did complain about hostility toward refugees and Muslims. In sharing a PBS link about the impact of immigration on the U.S. economy, he wrote, “Why refugees and immigrants are good for America.”

One post by Alissa said, “God created Adam and Eve not Adam and Steve just saying.”

Another post said “What Islam is really about” and listed virtues such as decency, humility and forgiveness.

In September 2019, when Alissa was 20, he posted “#NeedAGirlfriend.” The cousin in Syria said Alissa’s family was trying to find a wife for him, but without success.

The Facebook account slowed down dramatically after October 2019. There were only three publicly accessible posts in 2020, with the last in September.

In Merrill Middle School, one woman said Alissa “was the sweetest kid ever. Really quiet and respectful. He got along with everyone. And he talked to everybody. He would play with everyone.” She spoke on the condition of anonymity, fearing for her own safety.

“He never got bullied,” the woman recalled. “He was a good kid. I just don’t know what happened.”

A classmate in a 10th-grade English class remembered Alissa as almost painfully anonymous, alone enough that he and another student went out of their way to include him.

“In high school, he didn’t really have a lot of friends, and that’s why [the other student] and I tried to reach out and make him welcome,” said the classmate, who spoke on the condition of anonymity for fear of harassment on social media. “He was alone and he was just kind of shy.”

Dorokhov and other students remembered having lunch occasionally at one of the Alissa family’s restaurants, the Sultan Grill, not far from Arvada West High School. But little about Alissa himself stood out.

“I had a Volkswagen; we talked about my car,” said Dorokhov, an immigrant from Ukraine who said he works as a mechanic. “I know he talked about his parents’ restaurant. Also, I remember a black Mercedes, he told me how tinted his windows were.”

According to the arrest warrant affidavit, police found a black Mercedes sedan in the parking lot of the King Soopers market where the shooting occurred. The vehicle was registered to one of Alissa’s brothers.

Hernandez, the wrestler, said Alissa became increasingly anti-social each of the three years that he knew him in high school. One of Alissa’s brothers, Ali Aliwi Alissa, 34, told the Daily Beast in a phone interview that his brother was mentally ill and paranoid, adding that in high school he would talk about being chased, or that someone was looking for him. He added that his brother was bullied in high school.

Hernandez was a year behind the suspect in high school. The two competed on the same team for three years. Hernandez, who graduated in 2019, said the team was like a family. He was not close to Alissa, he said, but bonded with him through the sport, as teammates often do. He struggled to reconcile the teenager he remembered as “genuine and super nice” with the man accused of slaying 10 people.

He recalled, however, that Alissa’s mood could grow dark in an instant. Hernandez was aware Alissa had “some problems with anger.”

“If something made him mad, within a split second, he’d change,” Hernandez said. “When he got mad, it was scary, I won’t lie.”

About a month ago, Hernandez ran into Alissa at a restaurant and said he seemed “100 percent fine.” The two spoke for a few minutes and Hernandez asked how Alissa was doing.

“He said, ‘Yeah, I’m doing pretty good.’ He said life was tough with the covid situation, but that was pretty much it. He seemed happy.”

That interaction makes events even more difficult to understand, he said.

“I’m in shock,” he said, “and I’m sad about it.”

Jennifer Oldham in Denver and Souad Mekhennet, Julie Tate and Alice Crites in Washington contributed to this report.