The sight alarmed his family. The gun didn't look like a rifle featured in old Western movies, Alissa's sister-in-law told police, according to an arrest affidavit.
Rather, it looked like a "machine gun."
On Monday, police said, the 21-year-old man stormed a King Soopers supermarket in Boulder and killed 10 people with a spray of bullets.
Little is known about Alissa or what may have motivated him to open fire at the store. The 10 victims ranged in age from 20 to 65 and included Eric Talley, an 11-year veteran of the Boulder police force, who was among the first officers on the scene. Talley (51) was the father of seven children and had recently been looking for a less dangerous job, according to a statement released by his father.
Police identified the nine other victims 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 Jody Waters (65).
According to the court affidavit, Alissa had purchased a Ruger AR-556 semi-automatic pistol - a weapon that resembles a semi-automatic rifle and has a 30-round capacity - on March 16, six days before the shooting.
His sister-in-law, whose name was redacted from the court document, told investigators that she believed the gun his family had taken from Alissa was back in his room as of Monday night.
Alissa, a naturalised US citizen from Syria who graduated from Arvada West High School in 2018, has had at least two previous run-ins with the law, according to an Arvada Police Department spokesman.
There were criminal reports for a third-degree assault in 2017 and for criminal mischief in 2018, the department said.
The police department has not released records outlining the outcomes of those cases.
It was not immediately clear if Alissa had a lawyer in his current case, and family members did not respond to requests from Reuters for comment.
Ali Aliwi Alissa, the suspect's 34-year-old brother, told The Daily Beast that Alissa was antisocial and paranoid, and had talked at times in high school about "being chased" or said someone was looking for him.
Records from Arvada West High School show he was on the wrestling team during two seasons ending in 2018, according to Cameron Bell, a spokeswoman for the school district.
Alissa, who was treated for a leg wound suffered in an exchange of gunfire with responding police, is now in jail awaiting an initial court appearance on murder and other charges. Authorities said they were confident he acted alone.
Alissa has been released from a hospital, where he was treated for a leg wound suffered in an exchange of gunfire with responding police, and transported to county jail on Tuesday afternoon to await an initial court appearance on murder and other charges.
NOT THE FIRST MASS SHOOTING IN COLORADO
Colorado has been the site of some of the most shocking episodes of gun violence in US history, including the 2012 mass shooting at a movie theatre in Aurora and the 1999 rampage at Columbine High School near Littleton.
The latest bloodshed came only six days after a gunman went on a killing spree on March 16 in the Atlanta area, fatally shooting eight people at three day spas before he was arrested.
"My heart aches today, and I think all of ours does, as Coloradans, as Americans, for this senseless tragedy and loss of life," Governor Jared Polis said.
The two mass shootings are likely to intensify pressure on Democratic President Joe Biden to fulfill his campaign pledge to enact tougher gun limits. But legislation to ban assault-style weapons and tighten background checks have stalled amid Republican opposition in Congress.
"I don't need to wait another minute, let alone another hour, to take common-sense steps that will save lives in the future," Biden said at the White House on Tuesday. "This is not and should not be a partisan issue."
House of Representatives Speaker Nancy Pelosi and Senate Majority Leader Chuck Schumer, the Democratic leaders in Congress, said on Tuesday that the violence underscored the need for stricter gun laws.
Biden also ordered US flags to fly at half-staff in honor of the victims; they had just been raised at sunset on Monday after having been lowered following the Atlanta killings.
