PARKLAND, Fla. — Sometime after 1:30 a.m. in a hotel conference room, the law enforcement officer’s eyes — even before his words — told Tom and Gena Hoyer that their youngest child was gone, killed in a mass shooting at Marjory Stoneman Douglas High School.

They remember so little about the ride home. Almost as soon as they entered their Parkland house, Ms. Hoyer walked upstairs into Luke’s second-floor room, untouched since the morning hours when he had been getting ready for school. She sat on the edge of his unmade bed next to the night stand where he had left his prescription glasses. She was in Luke’s room at this hour because she believed that if she did not do this task right now, she might never.

Two thoughts entered her mind. One was clear but painful to contemplate: How could their family go on without Luke? The other was much less formed and so much harder to answer: What now? That question would crystallize over the months and years into something else: What does justice mean?

It’s been nearly four and a half years since an afternoon ambush at the high school on Valentine’s Day, 2018, claimed Luke and 16 other students and faculty members.

What the Hoyers now know is that the concept of justice shape-shifts with the tides of mourning. It is both evasive and precise, and at times, uniquely unsatisfying. It has forced them to think deeply about society’s legal prescription for certain convicted murderers, and to consider what the fallout of yet another mass shooting means for school safety and gun laws.

They came to think of justice more broadly, not just as the punishment of an individual but as their own power to try to build something meaningful from the tragedy by making schools safer.

“Justice is complicated,” Ms. Hoyer said. “I struggled with it.” What helped, she said, was viewing it as something that also exists “beyond the courtroom.”

Her husband put it this way: “We couldn’t allow ourselves to think of justice only in terms of this person being held accountable for what he did.”

Nikolas Cruz pleaded guilty in October 2021 to the murder or attempted murder of 34 people at the school in Parkland, but a jury still had to consider his sentence. Through week after week of jury selection, the concept of justice was what the Hoyers clung to in a 17th-floor courtroom in downtown Fort Lauderdale as they sat only a few feet away from the killer of their son. And it is what they hold on to now as the jury of seven men and five women considers whether the killer should be sentenced to life in prison without the possibility of parole, or be put to death.

On the first day of the sentencing trial last week, the lead prosecutor, Michael J. Satz, described the violence that was unleashed upon the high school, naming the victims one by one and the number of times each had been shot. Videos taken inside classrooms were shown to jurors, and though the audience could not see the video images, everyone in the courtroom could hear the audio of booming gunfire, screams and pleas for help.

For the Hoyers, it was hard and overwhelming — and necessary. They believe learning the whole truth, and having the world learn it too, is part of winning justice for their son.

“I felt like I had to hear it, because it fills out the picture for me a little more,” Mr. Hoyer said.

Each day in court, while painful, offers a fuller narrative of what happened, and a sense that every single second was critical.

“For such a long time, you are raising your kids, and that feels like your purpose,” Mr. Hoyer said, his voice thinning to a whisper. “And then suddenly, one day, one of your purposes is gone. In that void, we really thought about how to go on, and what justice meant to us. The truth is, I don’t know. I think, more than anything, I want fairness, if that makes sense.”

The sentencing trial is rare. In modern times, no American gunman who killed so many people in a single attack has survived to face trial until now.

Some of the Parkland families favor the death penalty. Others are against capital punishment and are prepared to accept a life sentence for the gunman. Still others have said they believed the killer deserved death but did not want to experience an emotionally brutal sentencing trial.

Just after the shooting, Ms. Hoyer thought a life sentence was the right and simplest path. But within months of the shooting, as they learned more details, both the Hoyers became convinced that death was the appropriate punishment for the man who killed their son. They knew that a sentencing trial would mean weeks or months of their lives fully embedded in the courtroom.

It would mean revisiting the last horrific moments of Luke’s life.

It would mean reliving what the passage of time might have finally softened.

It would mean contemplating the questions Ms. Hoyer uttered in the moments after the law enforcement officer confirmed Luke’s death: “Did my baby die alone, and did he suffer?”

That Valentine’s Day morning in 2018, Ms. Hoyer dropped Luke off at school. She can still visualize him walking across the street, wearing a hoodie and carrying his backpack. Mr. Hoyer had already left for work. Nothing about the day stood out.

But in the afternoon, the gunman, a former student with a history of emotional and behavioral problems, entered the high school armed with an AR-15-style semiautomatic rifle and fired in the hallways and through classroom doors. Luke was among the first to be hit in the rampage, shot twice in the first-floor hallway.

The hours that followed were a whirlwind of panic and dread and resignation: Ms. Hoyer furiously texted Luke. Mr. Hoyer raced home from his office in Miami. The couple separately searched hospitals in the area. They paced and prayed in a hotel conference room for hours before learning the fate of their son.

In October 2021, in a court hearing attended by the Hoyers and other families, Judge Elizabeth A. Scherer read each charge against the gunman aloud. Luke Hoyer’s was the first victim’s name she spoke. The gunman responded, “Guilty,” 34 times. Later in the hearing, he apologized for the attack.

The Hoyers had always thought of the death penalty the way most Americans do; they believed it was appropriate in certain murder cases. After the authorities laid out the details of the Parkland massacre — the planning, the multiple opportunities the gunman had to stop firing — the Hoyers were convinced this was one of those cases.

“This guy killed 17 people,” Mr. Hoyer said. “I think when he pulled that trigger, he gave up his right to humanity.”

For Ms. Hoyer, it was not just the death of her son that shaped her view, but also the scale of the attack. Too many people had died, she thought, for his life to be spared.

Earlier this year, the couple began taking time off from their jobs to attend the court proceedings. Mr. Hoyer, 59, retired a few weeks ago from his job as an executive of an in-home health care company; Ms. Hoyer, 58, works for a foster care agency. They typically attended court one or two days a week during jury selection, but now that the trial has begun, they will probably go more often. Always, Ms. Hoyer wears Luke’s old cross necklace, tucked out of sight. They had given it to him for Christmas, less than two months before his death.

Though the trial is only for sentencing, it is expected to stretch over several months. Defense lawyers will lay out any mitigating factors, like a troubled childhood and mental health issues, that might make the case for sending the gunman to prison for life rather than executing him. A death-sentence recommendation from the jury must be unanimous. If the gunman is sentenced to death, he would join more than 300 inmates on death row in Florida.

“Like the other families, we have had to deal with so much grief for so long,” Ms. Hoyer said. Referring to the couple’s daughter and surviving son, she said, “What we want is to make sure Abby and Jake are OK and we are being good parents to them. I feel like our future needs to be focused on them and their lives, while always remembering and loving our Luke. I have to believe he would want that, too.”

One of her greatest struggles, she said, has been finding a way to balance “seeing Luke to the finish line” with a trial that inevitably brings the harrowing details of her darkest day hurtling back. At times, the pain feels physically unbearable, burrowing in her stomach.

She sometimes finds relief in the quiet of Luke’s room, sitting on his bed as she had on that first night. There is something comforting about the space, which is nearly preserved, full of remembrance photos and plaques and jerseys, his old books and his backpack, which now has an evidence tag. There is a card from a mother whose son was killed in the Virginia Tech shooting 15 years ago.

The Hoyers have come to think of the Parkland shooting as more than a personal loss. It was the larger story of the intractability of American gun violence and school safety.

Not long after Luke died, they did something to reflect his interests: establishing the Luke Hoyer Athletic Fund, which pays for sports-related costs for foster children in Broward County.

Then they helped to form Stand With Parkland, an advocacy group representing most of the families who lost loved ones in the shooting. The Hoyers describe themselves as moderate Republicans who grew up around guns but don’t own any. Collectively, the group supports increasing mental health screening and support, school safety reform and responsible gun ownership, which includes “red-flag” laws that allow the authorities to take guns away from people who are shown to be a danger to themselves or others.

The couple considers the Bipartisan Safer Communities Act recently passed by Congress as a good first step. That package incorporates legislation — named for Luke and a classmate, Alex Schachter, who was also killed in the shooting — that establishes a federal clearinghouse to identify and share school safety best practices and recommendations.

Luke’s final resting place is in a cemetery in Broward County, chosen because it reminded Ms. Hoyer of her parents’ hometown, Joanna, S.C.

Luke Thomas Hoyer, 15, was the youngest of the family’s three children. The family called him “Lukey Bear.” He loved chicken nuggets, the Clemson Tigers and the Miami Heat. He spent the summer before his freshman year perfecting a new curly-top hairstyle. Just shy of six feet, Luke had gone through a growth spurt in both height and confidence. He loved basketball, but in the months before his death he had turned to football, and planned to try out for his high school team.

Luke’s parents are both comforted and shattered by ordinary memories — the way he watched ESPN every morning while eating his breakfast; or the way he raised one eyebrow when he was having an epiphany; or the long hours it took for him to mow the grass because he took so many breaks.

Sometimes the Hoyers are able to imagine what the future might have been like for Luke. They imagine the milestones that were just a few months and years away: a driver’s license; a high school prom; high school graduation (they received his diploma, awarded posthumously, last year); college admission; college graduation.

Now Luke comes to Ms. Hoyer in her dreams. Almost always, she is cupping his cheeks, telling him how much she loves him.