DENVER (AP) — Opening statements are scheduled Thursday in the trial of a mentally ill man who shot and killed 10 people at a Colorado supermarket in 2021.

Police say Ahmad Al Aliwi Alissa targeted people who were moving, both inside and outside the store in the college town of Boulder, killing most of them in just over a minute.

No one, including Alissa’s lawyers, disputes he was the shooter. Alissa, who was diagnosed with schizophrenia after the shooting, has pleaded not guilty by reason of insanity so the three-week trial is expected to focus on whether or not he was legally sane — able to understand the difference between right and wrong — at the time of the shooting.

Alissa 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.

Prosecutors will have the burden of proving he was sane, attempting to show Alissa knew what he was doing and intended to kill people at the King Soopers store.

Why Alissa carried out the mass shooting remains unknown.

RELATED COVERAGE Teen arraigned on attempted murder in shooting of San Francisco 49ers rookie says he is very sorry Prosecutor asks for a charge to be reinstated against Alec Baldwin in the ‘Rust’ case A 14-year-old student fatally shot 4 people in a rampage at a Georgia high school, officials say

The closest thing to a possible motive revealed so far was when a mental health evaluator testified during a competency hearing last year that Alissa said he bought firearms to carry out a mass shooting and suggested that he wanted police to kill him.

The defense argued in a court filing that his relatives said he irrationally believed that the FBI was following him and that he would talk to himself as if he were talking to someone who was not there. However, prosecutors point out Alissa was never previously treated for mental illness and was able to work up to 60 hours a week leading up to the shooting, something they say would not have been possible for someone severely mentally ill.

Alissa’s trial has been delayed because experts repeatedly found he was not able to understand legal proceedings and help his defense. But after Alissa improved after being forcibly medicated, Judge Ingrid Bakke ruled in October that he was mentally competent, allowing proceedings to resume.