The man who fatally shot 10 people in 2021 at a Boulder, Colo., grocery store was found guilty of murder on Monday, with jurors rejecting his lawyers’ claims that his mental illness made him unable to distinguish right from wrong.

The verdict followed about two weeks of testimony that focused on the mind-set of the gunman, Ahmad Al Aliwi Alissa, at the time of the shooting. Mr. Alissa, now 25, was diagnosed with schizophrenia after the attack; his lawyers admitted that he carried out the shooting but said that he was so unwell at the time that he could not know that what he was doing was wrong.

The shooting on March 22, 2021, took place at a King Soopers supermarket, less than a week after another mass shooting, in Atlanta. The victims, ranging in age from 20 to 65, included the first police officer to arrive at the scene, store employees and shoppers.

Jurors deliberated for about six hours over two days before coming to their unanimous conclusion. Having been convicted of 10 counts of first-degree murder, Mr. Alissa by law must be sentenced to life in prison without the possibility of parole. The judge said Mr. Alissa would be sentenced immediately after the verdict was read on Monday.

At closing arguments on Friday, Michael Dougherty, the Boulder County district attorney, said Mr. Alissa had planned his attack and been “determined” and “focused” as he entered the parking lot of the King Soopers store and began firing at his victims.

Mr. Dougherty said Mr. Alissa’s actions that day, including the fact his eventual surrender to the police, indicated that he knew what he was doing was wrong. He showed jurors evidence that Mr. Alissa had researched other mass shootings and noted the testimony of a psychologist who met with Mr. Alissa and said he seemed disappointed to learn that Colorado no longer imposes the death penalty.