A gunman who fatally shot 10 people at a grocery store in Colorado in 2021 has been found guilty of murder and could face life in prison.

On Monday, a jury rejected the defence’s argument that 25-year-old Ahmad Alissa should be found not guilty by reason of insanity.

The defence had argued that Alissa was diagnosed with schizophrenia and could not distinguish the meaning of his actions when he opened fire at the King Soopers grocery store in the city of Boulder.

“This tragedy was born out of disease not choice,” defence lawyer Kathryn Herold told the jury during closing arguments.

District Attorney Michael Dougherty, meanwhile, argued that the nature of the attack showed Alissa was intentional in his actions.

“He is methodical and he is brutal,” Dougherty told jurors.

Whether Alissa was responsible for the shooting and the details of the attack were never in question during the trial, which began earlier this month.

Alissa had started shooting within moments of arriving in the car park of the store, killing three people before heading inside. He chased several of those he shot and sought out others who were hiding.

Prosecutors pointed to those decisions as evidence that Alissa was acting sanely during the attack. They also argued that the illegal magazines and steel-piercing bullets Alissa carried showed the attack was deliberate.

State forensic psychologists said that Alissa’s fear of being arrested or killed by the police showed he was sane at the time of the killings. Still, psychologists said they could not provide full confidence in their finding – a point seized on by the defence.

Alissa had repeatedly told the psychologists he heard what he described as “killing voices”, but he did not provide further details. Alissa’s family also reported that he had become withdrawn and spoke little, and that he had become increasingly paranoid and heard voices in the years leading up to the attack. They said he had not received any mental health treatment prior to the attack.

The state forensic psychologists also concluded that voices likely played a role in the attack and that they did not believe it would have happened if he did not have a mental illness.

Still, Colorado law draws a distinction between mental illness and insanity. It defines the latter as having a mental disease so severe that it is impossible for a person to tell right from wrong.

The verdict capped a trial filled with harrowing testimonies from survivors of the attack.

One survivor, an emergency room doctor, said she crawled onto a shelf and hid among bags of potato chips.

A pharmacist at the grocery store testified that she heard Alissa say, “This is fun” at least three times as he fired throughout the store with a semi-automatic pistol resembling an AR-15 rifle.

Prosecutors said Alissa, who was born in Syria and emigrated to the US with his family as a small child, had researched locations for possible attacks. However, they did not provide any other motive.