The letters came streaming in: from battalion brothers who had served alongside Christopher Pelkey in Iraq and Afghanistan, fellow missionaries and even a prom date.

A niece and nephew addressed the court.

Still, the voice that mattered most to Mr. Pelkey’s older sister, Stacey Wales, would most likely never be heard when it was time for an Arizona judge to sentence the man who killed her brother during a 2021 road rage episode — the victim’s.

Ms. Wales, 47, had a thought. What if her brother, who was 37 and had done three combat tours of duty in the U.S. Army, could speak for himself at the sentencing? And what would he tell Gabriel Horcasitas, 54, the man convicted of manslaughter in his case?

The answer came on May 1, when Ms. Wales clicked the play button on a laptop in a courtroom in Maricopa County, Ariz.

A likeness of her brother appeared on an 80-inch television screen, the same one that had previously displayed autopsy photos of Mr. Pelkey and security camera footage of his being fatally shot at an intersection in Chandler, Ariz. It was created with artificial intelligence.

“It is a shame we encountered each other that day in those circumstances,” the avatar of Mr. Pelkey said. “In another life, we probably could have been friends. I believe in forgiveness and in God, who forgives. I always have and I still do.”

While the use of A.I. has spread through society, from the written word to memes and deepfakes, its use during the sentencing of Mr. Horcacitas, who got the maximum 10 and a half years in prison, appeared to be uncharted.

It reverberated far beyond the courtroom, drawing headlines, questions and debate. Critics argued that the introduction of A.I. in legal proceedings could open the door to manipulation and deception, compounding the already emotional process of giving victim impact statements.

One thing was certain: The nearly four-minute video made a favorable impression on the judge, Todd Lang, of the Maricopa County Superior Court, who complimented its inclusion moments before sentencing Mr. Horcasitas.

“I loved that A.I.,” Judge Lang said, describing the video’s message as genuine. “Thank you for that. And as angry as you are, and justifiably angry as the family is, I heard the forgiveness. And I know Mr. Horcasitas appreciated it, but so did I.”

Much in the same way that social media apps have been placing labels on A.I.-generated content, the video opened with a disclaimer.

“Hello, just to be clear, for everyone seeing this, I am a version of Chris Pelkey recreated through A.I. that uses my picture and my voice profile,” it said. “I was able to be digitally regenerated to share with you today.”

While many states provide an opportunity for victims and their families to address the court during sentencings, some are more restrictive in the use of video presentations and photographs, according to legal experts.

But victims have broader latitude in Arizona. Ms. Wales said in an interview on Wednesday that she had discovered that fact as she bounced the idea of using A.I. off a victims’ rights lawyer who represented Mr. Pelkey’s family.

“She says, ‘I don’t think that’s ever been done before,’” Ms. Wales said.

Ms. Wales had been preparing her victim’s impact statement for two years, she said, but it was missing a critical element.

“I kept hearing what Chris would say,” she said.

Ms. Wales said that she then enlisted the help of her husband and their longtime business partner, who had used A.I. to help corporate clients with presentations, including one featuring a likeness of a company’s chief executive who had died years ago.

They took Mr. Pelkey’s voice from a YouTube video that they had found of him speaking after completing treatment for PTSD at a facility for veterans, she said. For his face and torso, they used a poster of Mr. Pelkey from a funeral service, digitally trimming his thick beard, removing his glasses and editing out a logo from his cap, she said.

Ms. Wales said that she had written the script that was read by the A.I. likeness of her brother.

“I know that A.I. can be used nefariously, and it’s uncomfortable for some,” Ms. Wales said. “But this was just another tool to use to tell Chris’s story.”

Vanessa Ceja-Cervantes, a spokeswoman for the Maricopa County attorney, said in an email that the office was not aware of A.I. being used before to give a victim’s impact statement.

Jason D. Lamm, a defense lawyer for Mr. Horcasitas, said in an interview that it would have been difficult to block the video from being shown.

“Victims generally have extremely broad latitude to make their voices heard at sentencing, and the rules of evidence don’t apply at sentencing,” Mr. Lamm said. “However this may be a situation where they just took it too far, and an appellate court may well determine that the court’s reliance on the A.I. video could constitute reversible error and require a resentencing.”

Ms. Wales emphasized that the video of her brother’s likeness was used during only the sentencing phase of the case, not in either of Mr. Horcasitas’s two trials. Both ended with convictions. He was granted a second trial because prosecutors did not disclose certain evidence during the first, according to court records.

On Nov. 13, 2021, Mr. Pelkey was stopped at a red light in Chandler when Mr. Horcasitas pulled up behind him and honked at him, prompting Mr. Pelkey to exit his vehicle and approach Mr. Horcasitas’s Volkswagen and gesture with his arms as if to say “what the heck,” according to a probable cause statement. Mr. Horcasitas then fired a gun at him, hitting Mr. Pelkey at least once in the chest.

Cynthia Godsoe, a professor at Brooklyn Law School and a former public defender who helps write best practices for lawyers for the American Bar Association, said in an interview on Thursday that she was troubled by the allowance of A.I. at the sentencing.

“It’s clearly going to inflame emotions more than pictures,” Ms. Godsoe said. “I think courts have to be really careful. Things can be altered. We know that. It’s such a slippery slope.”

In the U.S. federal courts, a rule-making committee is currently considering evidentiary standards for A.I. materials when parties in cases agree that it is artificially generated, said Maura R. Grossman, a lawyer from Buffalo who is on the American Bar Association’s A.I. task force.

Ms. Grossman, a professor at the School of Computer Science at the University of Waterloo, who also teaches at the Osgoode Hall Law School, both in Canada, did not object to the use of A.I. in the Arizona sentencing.

“There’s no jury that can be unduly influenced,” Professor Grossman said. “I didn’t find it ethically or legally troubling.”

Then there was the curious case of the plaintiff in a recent New York State legal appeal who made headlines when he tried to use an A.I. avatar to make his argument.

“The appellate court shut him down,” Ms. Grossman said.