CHESAPEAKE After 27 years in the Navy, Harley Peterson couldn’t help but evaluate a passing car as he would an unfamiliar ship cruising past his own off the coast of Vietnam or in the Atlantic.

On a late October night in 2002, a dark blue Chevrolet Ca- price caught his attention. Its direction was north, presumably headed back to New Jersey, which the license plate flagged as the driver’s home state.

The two guys in the Chevy probably are heading home after visiting family down South, thought Mr. Peterson, who hails from Pompano Beach, Fla. He wondered whether the driver and passenger might be bringing back some butter beans.

But Mr. Peterson quickly realized something was odd: The Chevy’s occupants seemed agitated. A friendly nod on his part didn’t help much — a sign from his military career that led him to believe the other vehicle was likely more foe than friend.

The two cars were speeding along Interstate 95 north of Richmond. The Caprice pulled in front of Mr. Peterson; he remembers seeing a hole in the vehicle’s back panel. It was large enough for a hand. Or the barrel of a rifle.

It was all coming together — to borrow phrasing from Mr. Peterson’s therapist years later — like tumblers in a lock clicking into place.

Mr. Peterson said he could see the passenger crawling into the back seat, with easy access to the trunk. In that moment, Mr. Peterson said he realized that the men driving alongside him were the snipers who had been terrorizing the region for weeks.

For the next several miles and minutes, Mr. Peterson recalled, his car and the Caprice engaged in a tense chase on I-95 near Ashland. Mr. Peterson believes the blue sedan was trying to maneuver to shoot at him. Then suddenly, the Caprice exited on U.S. 301.

Although no shots were fired, Mr. Peterson said he was sweating bullets. He said he immediately used his cell phone to call the Virginia State Police on #77, the line for reporting reckless and dangerous drivers.

State Police were unable to produce records related to the report; the agency doesn’t have #77 calls from that far back, a spokeswoman said.

After the ordeal on I-95, Mr. Peterson said he told his supervisor at the Washington Navy Yard about the encounter. Attempts to contact the supervisor, who has since retired, were unsuccessful.

But Mr. Peterson said he is sure of what he saw. He said the vehicle he reported had the same license plate number that, a few days later, authorities warned the public to look out for: NDA 21Z. On Oct. 24, 2002, police surrounded a Chevy Caprice bearing those plates and arrested John Allen Muhammad, 41, and Lee Boyd Malvo, 17 – “The D.C. Snipers.”

Mr. Peterson never went public with his experience. He said he has been wrestling with questions, sometimes wondering whether going to the media immediately could have saved the life of the snipers’ last of 10 victims. Now, after years of staying quiet, Mr. Peterson has decided to tell his story.

A month of terror from D.C. to Richmond

In October 2002, Mr. Muhammad and Mr. Malvo went on a shooting spree in the Washington area, killing 10 people and critically injuring three. A hole had been cut in the rear panel of the Caprice and that served as a firing point for the sniper rifle.

Mr. Peterson remembers the month as a tense time because of the shootings. He said he doesn’t recall the exact date of the night he encountered the men he believes were Mr. Muhammad and Mr. Malvo as he was driving to his job at the Washington Navy Yard from his home in Chesapeake. It was probably Oct. 19 or 20, said Mr. Peterson, who had an apartment in Alexandria where he stayed during the workweek.

But other details of the run-in are still clear 17 years later. “Muhammad and Malvo attempted to take my life on a night in late October 2002,” Mr. Peterson later wrote.

“I think I remember everything,” Mr. Peterson said during an interview in the living room of his suburban Chesapeake home. When Mr. Peterson first spotted the Caprice, he recalls feeling a sense of relief. “Actually, it was kind of a welcome sight to see a couple of able young African-Americans and me out there,” said Mr. Peterson, who also is African-American. “There’s safety in numbers.”

But tensions grew between the drivers. It started with what Mr. Peterson perceived as the pair blowing off his salutation. Then they seemed to show an increased interest in him. Too often, they ended up next to him in the flow of traffic. So Mr. Peterson flipped them off.

Shortly after, Mr. Peterson believes he saw Mr. Malvo stroking Mr. Muhammad’s neck and shoulders. So Mr. Peterson made a kissing gesture, which he now sees as homophobic, toward the Caprice.

“I say that now in all awareness that that was 2002, and we’ve all looked at things like that a lot differently,” Mr. Peterson said. “But that’s what I did at the time.”

Mr. Malvo, a native of Kingston, Jamaica, had entered the U.S. illegally alongside Mr. Muhammad. In a 2012 interview on NBC’s Today show, Mr. Malvo alleged that Mr. Muhammad had sexually abused him since he was about 15.

“I felt a sense of shame, and I just said, ‘That’s just something that I’d never tell anyone,’ ” Mr. Malvo said in the television interview. “And to a certain extent, up until that point, I really couldn’t handle it.”

After Mr. Peterson made the kissing gesture, both cars started speeding up and shifting lanes. Mr. Peterson said the Chevy seemed intent on staying in front of him.

It was at this moment that he feared the occupants of the Caprice might attempt an open- highway shooting. The passenger Mr. Peterson believes to have been Mr. Malvo remained in the backseat — where he could access the trunk — while speaking with the driver.

“I would probably not believe it myself if it hadn’t happened. But it was almost like I could see the nostrils flare in their breath- ing,” Mr. Peterson said. “I don’t know whether it’s adrenaline or what, but I had a very good look at both of them, and a lot of this took place almost like it was in slow motion.”

Just as quickly as the interaction began, the Chevy pulled alongside Mr. Peterson — driver and passenger laughing — and exited I-95 north of Richmond at Bowling Green in Caroline County.

Mr. Peterson knew the State Police often patrolled the area because he’d been pulled over for speeding in the past. So he said he dialed #77 and made a report about the incident.

“I provided a description of the vehicle, license plate num- ber, direction headed based upon their exit from the interstate highway, and a description of the vehicle’s occupants, including estimates of age and racial identities as African-Americans,” Mr. Peterson said in a written statement to the Capital News Service before the interview.

“My report importantly contained a caution to State Police leadership that a stop of the killer’s vehicle by a single patrol car and officer be avoided at all cost due to the likelihood of a loss of life to one of their own. I described the method of the shooter accessing the trunk of the vehicle from the passenger compartment and firing from an opening fashioned in the car’s rear panel in complete concealment. I believed the threat to an officer’s life would be imminent in such a case.”

Until that point in the investigation, police had told the public to be on the lookout for a white van possibly connected with the attacks. On Oct. 23, 2002, authorities updated the vehicle’s description, saying they were looking instead for a blue Chevy Caprice. Police had received reports about the Caprice as early as Oct. 3, 2002. It is unknown whether Mr. Peterson’s report prompted authorities to revise their public warning.

Mr. Peterson’s instincts about the occupants of the Caprice were rooted in his military career. He applied his observation skills as he did when he was on an ammunition ship “provid- ing ammunition to do a lot of the carnage that took place in Vietnam.” Not a day went by that he didn’t realize the gravity of what he was doing.

Joining the military was a way for Mr. Peterson to see the world and to get a break in life. He said that he grew up in a housing project in Florida and that his mother died when he was in his teens. Mr. Peterson said he was surrounded by violence.

“I don’t know when I didn’t know that people killed other people,” Mr. Peterson said. “I don’t know that today there isn’t some 8-year-old on the South Side of Chicago that hasn’t seen the same things I saw growing up.”

Now 34, Mr. Malvo has been back in court

Following the pair’s arrest, Mr. Muhammad was charged with capital murder, terrorism and other crimes. Because of public- ity that could have influenced jurors, Mr. Muhammad’s trial was moved from Northern Virginia to Virginia Beach. He was convicted in November 2003, sentenced to death and executed six years later.

Mr. Malvo was tried as an adult in Chesapeake in December 2003, at a courthouse less than 10 miles from where Mr. Pe- terson lives. He was found guilty of capital murder, terrorism and illegal use of a firearm and sentenced to life without parole. He is incarcerated at Red Onion State Prison, a super-maximum security facility in Wise County.

Since Mr. Malvo was sentenced for crimes he committed at age 17, legal opinions on capital punishment and life sentences for minors have shifted.

In 2005, the U.S. Supreme Court ruled that juveniles can- not receive the death penalty. In 2010, the court prohibited life sentences for minors convicted of crimes other than homicide.

And most pertinent to Mr. Malvo’s case, the court ruled in 2012 that juveniles cannot receive life without parole unless their crime “reflects irreparable corruption.” In 2016, the high court made that decision, called Miller v. Alabama, retroactive.

The rulings stem from a new understanding of how young brains develop. Dr. Hayley Cleary, a Virginia Commonwealth University professor who studies adolescent development in legal contexts, said the decision-making parts of the brain don’t finish developing until the mid-20s.

“The prefrontal cortex of the brain, which is this front part of the brain that governs rational decision-making and impulse control and strategic thinking, is the last part of the brain to develop,” Dr. Cleary said.

As a result, courts are reassessing how to handle teenagers who commit crimes. Currently, Dr. Cleary said, “We treat them like an adult. We process them in the adult court system. There’s a major disconnect there.”

In October, the U.S. Supreme Court heard arguments on whether to grant Mr. Malvo a new sentencing hearing. The outcome won’t change much for him — he already has 10 life sentences — but the results could matter for other people sen- tenced to life without parole as minors in Virginia.

The core of the case is whether Virginia’s sentencing sys- tem is discretionary or mandatory for capital murder. The state argues that it is discretionary because the judge could have reduced the jury’s recommendation for life without parole in Mr. Malvo’s case.

But Mr. Malvo’s lawyers said the system is not truly discretionary. They said there has never been a case in which a juvenile received less than life without parole for capital murder.

Victims and their family members are split on Mr. Malvo’s bid for a rehearing.

A U.S. Supreme Court brief filed on behalf of the Maryland Crime Victims’Resource Center argues that reopening the criminal case and resentencing Mr. Malvo would retraumatize victims.

“These federal proceedings are not ‘no cost’ hearings for crime victims,” the brief states. “They are anything but ‘no cost’ hearings and they inflict a great emotional and psychological toll on the victims.”

However, the Philadelphia-based Juvenile Law Center, a nonprofit law firm that advocates for youths, wants a blanket ban on sentencing juveniles to life without parole. The center notes that 22 states already prohibit such punishments.

Mr. Peterson is firm in his belief that Mr. Malvo should remain in prison. He even wishes Mr. Malvo had met the same fate as Mr. Muhammad.

“The law is what it is, the rules are what they are, the courts are who they are. And that’s beyond anything I can control,” Mr. Peterson said. “But were it possible to have had the outcome that I sought, then Malvo would have been executed as well.”

Lasting trauma and survivor’s guilt

Mr. Peterson kept the story from his family for years. Almost three decades in the military teaches you to compartmentalize, he said.

“I think it was three years” before he told a family member — his daughter.

“Gosh, I’m sorry I have to change that. It was five years,” Mr. Peterson continued.

He paused, recalling his daughter’s age.

“I’m sorry, that makes it eight years,” Mr. Peterson said with a deep pause. “Eight years.”

Stephanie Peterson, who now works as an engineer, had just finished plebe summer at the Naval Academy in 2010 when her father recounted the story to her. The 27-year-old remembers her father telling her that he noticed the hole in the back panel of the Caprice, which is when he put two and two together.

“That’s a really big thing to have happened to my dad when we didn’t know about it,” she said. “He’s a very stoic person. He’s really reserved. He’s not boastful about things.”

Ms. Peterson said that whenever her father tells a story about his life, he uses it as a teachable moment. She noted his vigilance and tendency to notice small details as the factors that led to his realization about the snipers — a conclusion that might have eluded a less observant person.

As the 17th anniversary of the sniper attacks approached, Mr. Peterson said he learned of Virginia Commonwealth University’s student-run Capital News Service. He said he approached CNS with his story because “journalism school students are likely not that much older than Malvo was at the time of his killings. Intriguingly, one wonders what they might bring” to the story.

His therapist wanted him to tell family members to relieve some of the burden of the memory, and so he did. He was also asked to get rid of some of the physical memories of the encounter, like the flip phone he used to make the call and the car he was driving that night, which still sits in the driveway of his home.

Mr. Peterson, who retired last December, hasn’t gotten rid of anything from that night. Perhaps subconsciously, he considers those artifacts as trophies. They show he escaped unharmed from the encounter with the men he believes to be the snipers, his therapist told him.

“Well, I had never thought about it being a trophy,” Mr. Peterson said of his 2002 Nissan Maxima. “But after I did, I knew I wasn’t going to get rid of it. And that’s why it’s sitting out there right now.”

For a long time, memories of that night in October 2002 stayed with Mr. Peterson, and minuscule things triggered him, bringing him back to I-95. But as time went on — and Chevy Caprices began to disappear from the roads, along with news coverage of the shootings, save for a few anniversary stories — things started to get better.

But Mr. Peterson still feels deep regret over the death of the snipers’ final victim, Conrad Johnson, a 35-year-old bus driver. Mr. Johnson, who left behind a wife and two sons, was shot and killed standing at the top of the steps of his morning commuter bus in Aspen Hill, Md., on Oct. 22, 2002, a few days after Mr. Peterson crossed paths with the Chevy Caprice.

Mr. Peterson wonders if there was something he could have done to save Mr. Johnson, like making a more public report or going to the media. Maybe that way, Mr. Johnson would have known what to watch out for and who was a threat, Mr. Peterson said.

“Had I chosen another path, I can’t be sure he wouldn’t have died anyway. To the extent that that happened, I know I’ll always be troubled by that,” Mr. Peterson said. “It was hard to shake. It really is.”