A man who gunned down ten people inside a Colorado supermarket claimed he was having 'fun' when he perpetrated his terrifying rampage, it was revealed. 

Survivors of the 2021 shooting inside a King Soopers grocery store in Boulder, Colorado gave testimony in court on Thursday about the chilling demeanor of gunman Ahmad Al Aliwi Alissa. 

Pharmacist Sarah Chen said she heard the comments while hiding from Alissa alongside her coworkers.

'I heard him screaming, "This is fun. This is fun," she told the court.  

After years of turmoil in the courts over his mental state, Alissa's trial began last week and was expected to finish at the end of the month.

Alissa has pleaded not guilty by reason of insanity in the shooting. Despite his plea, nobody, including his own lawyers, have disputed that he was the shooter. 

Following the horrific massacre, Alissa was diagnosed with treatment-resistant schizophrenia. 

His attorneys say he had hallucinations that included hearing screaming voices, seeing people who weren't there and believing he was being followed. 

They argued that he should be found not guilty because he was not able to tell the difference between right and wrong at the time of the shooting. 

He is charged with 10 counts of first-degree murder, multiple counts of attempted murder and other offenses, including having six high-capacity ammunition magazine devices banned in Colorado after previous mass shootings.

Alissa allegedly began firing outside the grocery store, shooting at least one person in the parking lot before moving inside, employees told investigators. 

Employees and customers scrambled to escape the violence, some leaving through loading docks in the back and others sheltering in nearby stores.

Investigators say he researched how to carry out a mass shooting before he launched his own attack. 

They also say he targeted moving people, killing most of the 10 victims in just over a minute using a gun with a high-capacity magazine.

The victims were identified as; Denny Strong, 20, Neven Stanisic, 23, Rikki Olds, 25, Suzanne Fountain, 59, Terri Leiker, 51, Eric Talley, 51, Kevin Mahoney, 61, Lynn Murray, 62, Jodie Waters, 65, and Tralona Bartkowiak, 49.

Talley was a cop who recently changed jobs to work more away from the front-line when he was murdered. He was also a father-of-seven.

Leiker had worked at the grocery store for 30 years. She was dating a colleague who survived the shooting.

Witnesses described him shooting two of three shots then stopping calmly before opening fire again. One survivor said he was not 'spraying'.

Last October a judge ruled that Alissa was fit to stand trial and allowed the stalled prosecution to move forward. 

In a ruling, Judge Ingrid Bakke said she was convinced that, following forced medication, Alissa had a 'far improved capacity to elucidate his reasoning and decision-making'.