Opening statements begin in the trial of a Colorado man accused of killing 10 people at a King Soopers grocery store in 2021.

A jury will hear opening statements Thursday in the long-delayed trial of a Colorado man accused of killing 10 people at a Boulder grocery store more than three years ago.

Ahmad Al Aliwi Alissa faces 10 counts of murder, 38 counts of attempted murder and multiple other charges after a judge ruled last year that he was competent to stand trial in connection with the March 22, 2021, mass shooting at a King Soopers. He pleaded not guilty by reason of insanity last November.

Days before the massacre, Alissa, after passing a background check, was able to purchase the Ruger AR-556 pistol used in the killings, according to court documents and the owner of an Arvada, Colorado, gun shop.

The mass shooting occurred in a state that has seen similar tragedies, including the 1999 Columbine High School massacre in which 12 students and a teacher were killed, and the 2012 Aurora movie theater shooting in which 12 people were killed.

A district court judge in 2021 initially ruled Alissa incompetent to stand trial after he was examined by a defense expert, two doctors from a state hospital and a doctor chosen by prosecutors, according to court documents filed by the Boulder County District Attorney’s Office. Doctors said Alissa’s condition worsened while in Boulder County Jail. He was later sent to a state hospital for treatment.

However, last October a judge ruled he was competent to testify, noting that although Alissa had been diagnosed with schizophrenia, tests did not show he suffered from delusions that would have affected his ability to attend the trial.

Colorado District Court Judge Ingrid Bakke at the time “strongly urged” the Colorado Mental Health Institute in Pueblo to hold him because of the “seriousness of this case.” The state hospital granted the request. Alissa stayed in the hospital for more than a year before his condition began to improve when he resumed taking antipsychotic medication, as Bakke ordered.

On July 30, Bakke granted a defense motion to return Alissa to the Boulder County Jail. His lawyers argued that keeping him in Pueblo would deprive him of “the opportunity to understand the nature and purpose of these proceedings.”

Noting Alissa’s right to “consult with counsel” and “meaningfully participate” in his defense, his lawyers wrote, “The scale of this case is clear and obvious.”

CNN has reached out to his defense attorneys for comment.

Victims’ families are frustrated by delays in the case, a prosecutor told the court last summer, according to KMGH. And more than three years after the bloodshed, a clear motive continues to elude authorities.

On the afternoon of the shooting, Boulder 911 received multiple calls, according to a court affidavit. One caller told dispatchers that the shooter had fired through a car window and was chasing a man toward the street. Others said the shooter was wearing an “armored vest.”

Multiple callers said they were hiding in a grocery store. Employees told dispatchers they “saw the suspect shoot an elderly man in the parking lot. The suspect then approached the elderly man, stood over him and fired several additional shots at him,” the affidavit said.

Among those killed was 51-year-old Boulder police officer Eric Talley, who was among the first to respond to the call, former Boulder Police Chief Maris Herold said at the time. Witnesses told dispatchers the shooter was firing at police, the affidavit said. Officers exchanged gunfire with Alissa inside the store, Herold said.

The other victims included store manager Rikki Olds, 25, store employee Denny Stong, 20, store employee Teri Leiker, 51, Neven Stanisic, 23, Tralona “Lonna” Bartkowiak, 49, Suzanne Fountain, 59, Kevin Mahoney, 61, Lynn Murray, 62, and Jody Waters, 65.

Witnesses described chaos and a struggle to survive in the store’s aisles. Some customers fled through a back door to an employee area, where employees helped them navigate the unfamiliar space to safety. Others hid in storage rooms. A man waiting for a Covid vaccine grabbed his two young daughters and hid in a coat closet. A pharmacy technician hid under a desk and called his family to tell them he loved them.

According to Alissa’s testimony, at the time of his arrest, less than an hour after the initial 911 calls, he “took off all of his clothes and was wearing only shorts.” He suffered a gunshot wound to his upper right thigh.

Alissa’s family emigrated from Syria, her 34-year-old brother, Ali Aliwi Alissa, told CNN at the time. The brother said Alissa was paranoid and often believed she was being followed. The suspect has spent most of his life in the United States.

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