Boulder (United States) (AFP) - Ahmad Alissa's chubby face in an undated photo released by police Tuesday contrasts with images of his arrest as the suspect in the Boulder, Colorado mass shooting -- that of a shirtless, shoeless man with blood covering one leg escorted away by two officers.

Alissa, 21, faces 10 counts of murder in the first degree in the shooting, police said.

This is what we know about him up to now.

Still no known motive

Alissa was hospitalized and was in stable condition after being shot in the leg in an exchange of gunfire with police during the Monday attack, Boulder Police Chief Maris Herold told reporters.

By Tuesday afternoon he was well enough to be booked into jail, Boulder County prison officials said.

Police said they have not established a motive.

Alissa was armed with an AR-15 military-style assault rifle, US media reported, a weapon often used in mass shootings.

More weapons were retrieved from his home, CNN reported.

A fan of martial arts and computers

Alissa -- a resident of a suburb of Denver, the city located next to Boulder -- "lived most of his life in the United States," police said.

According to US media, citing a non-verified deleted Facebook account in his name, Alissa was born in Syria in 1999 and moved to the United States in 2002.

The account user described himself as a fan of martial arts and wrestling, along with postings about Islam, criticisms of former president Donald Trump, and some homophobic messages.

Alissa's brother Ali told The Daily Beast online magazine that he was "really sorry" for the victims.

Ali portrayed his brother as being "paranoid" and "anti-social."

"When he was having lunch with my sister in a restaurant, he said, 'People are in the parking lot, they are looking for me.' She went out, and there was no one. We didn't know what was going on in his head," Ali said.

Newsweek published excerpts of Facebook postings from what was reportedly Alissa's personal account -- which the social media platform quickly took down once he was named as the main suspect.

In a March 2019 posting Alissa said he believed that "my old school ... was hacking my phone."

Months later, in July, he posted that he "probably could" have a "normal life" if "these racist islamophobic (sic.) people would stop hacking my phone."

Ali was convinced that there was no political motive behind his brother's murderous rampage. "It's mental illness," he told The Daily Beast.

Formerly an "outgoing kid ... after he went to high school and got bullied a lot, he started becoming anti-social," the suspect's brother said.

Alissa's first encounter with police began in high school. He was arrested in 2017 after punching a classmate in the head without warning, local TV station KDVR reported.

To justify the attack Alissa, then 18, claimed that the victim had "made fun of him and called him racial names weeks earlier."

In 2018 Alissa pleaded guilty to assault in connection with this case, KDVR said.