AMNA NAWAZ: On this Memorial Day, nearly 81,000 American service personnel remain missing from previous war, the vast majority lost eight decades ago in World War II.

But there's now new cutting-edge technology to identify remains once thought unidentifiable.

And, as Nick Schifrin reports, it's allowing the U.S. military to fulfill its promise to leave no one behind.

NICK SCHIFRIN: In the town of Oregon, Ohio, this weekend, a funeral 80 years in the waiting.

Nobody here has ever met the man they call Uncle Jack, Staff Sergeant Jack Coy.

But his great-grandniece, Shawnelle Johns, clutches at his memory.

SHAWNELLE JOHNS, Great Niece of Staff Sgt.

Jack Coy: He was a very handsome man.

My great-grandmother and my grandmother and my great-aunt, they have always talked about him, always.

I feel I know him, even though I never got to meet him.

NICK SCHIFRIN: In 1941, at 18 years old, Coy graduated from high school and enlisted in the U.S. Army Air Forces.

He wanted to become a pilot, as he wrote home from boot camp.

SHAWNELLE JOHNS: "P.S., future pilot, I hope."

NICK SCHIFRIN: He ended up a gunner on American bombers.

And on February 24, 1944, he was on what was supposed to be his last mission aboard a B-24J Liberator... NARRATOR: American fighting planes with some of their spectacular missions deep into Germany.

NICK SCHIFRIN: ... dropping thousands of pounds of bombs on Germany.

But the plane was hit by German fire over Bad Salzungen.

Two crew parachuted out and survived.

Coy and five others were never heard from again, their remains never identified.

SHAWNELLE JOHNS: It still has his beard here in it.

NICK SCHIFRIN: For years, all that remained passed down to Johns and her aunt, Beverly Hahn, a small collection of his personal belongings.

SHAWNELLE JOHNS: We have lots of photographs of him in his uniform.

We have a lot of his pins and medals that he had on his uniform.

We have lots of letters.

NICK SCHIFRIN: Including one written to Coy by his mother just weeks before he died ahead of his 21st birthday.

SHAWNELLE JOHNS: "We're here hoping to see you by your birthday.

Oceans of love, mom."

And his birthday was in July.

Wow.

It's really hard to read.

NICK SCHIFRIN: But then, more than 70 years after he enlisted and his plane crashed, Johns' mother, Jack Coy's niece, received a letter before she died in 2020 that would bring nostalgia to life.

SHAWNELLE JOHNS: The Army contacted my mom and wanted to know if we could give DNA, because they're still trying to identify remains back from World War II of the soldiers that were lost.

And I looked at her and I said: "We need to do this.

We need to bring him home."

TIM MCMAHON, DNA Operations Director, Armed Forces Medical Examiner: It's amazing the honor that it is that we're helping them bring their loved one home.

NICK SCHIFRIN: Tim McMahon is the operations director of the Armed Forces Medical Examiner system's DNA Identification Lab.

And using new technology, they identified a handful of Coy's remaining bones.

TIM MCMAHON: Every one of these missing service members is a fallen hero.

They gave up their life to defend what we believe in.

And so the least that we can do is honor them by trying to continue to push the science to bring them home.

NICK SCHIFRIN: The scientists who work in the Dover, Delaware, lab use cutting-edge techniques that are often five to seven years ahead of state and local crime labs.

WOMAN: The alleged Golden State Killer who terrorized... NICK SCHIFRIN: And much like DNA technology that solves decade-old cold cases, their lab uses technology to identify decades-old service members' remains.

What conditions do these remains sometimes arrive in?

TIM MCMAHON: Mostly the stuff that we get from World War II, Korea and Vietnam, even though they have been in the environment for 50 to 80 years, our methods have been optimized to get those.

NICK SCHIFRIN: Next-generation sequencing uses magnets to allow scientists to extract human DNA in cases with degraded or little DNA material to work with.

TIM MCMAHON: It allowed us to enrich for the human DNA to get a result.

And, because of that, since 2016, we have 162 identifications of fallen heroes that would not have been made without that new methodology online.

KELLY MCKEAGUE, Director, Defense POW/MIA Accounting Agency: We as a country have an obligation.

It's a sacred promise made, not just to the service member, but, more importantly, to their families, who long for answers.

NICK SCHIFRIN: Kelly McKeague is the director of the Defense POW/MIA Accounting Agency.

Using anything they can find, even buttons, they recover, identify and repatriate Americans missing from every conflict from World War II to Operation Iraqi Freedom.

His agency recovers remains through field work like this 2015 operation in Vietnam's Dien Bien province.

Among his biggest challenges, 1,576 U.S. service members still missing in Vietnam.

KELLY MCKEAGUE: Witnesses are aging.

Witnesses are dying.

Having access to those firsthand witnesses, the fact that they were jets, that, when they impacted the ground, they were coming in at 600 miles an hour, the soil acidity in Vietnam has the pH of a lemon.

So, oftentimes, our teams are only able to find teeth or degraded bones.

NICK SCHIFRIN: And so, in that sense, I guess it's safe to say time is of the essence, especially with... KELLY MCKEAGUE: Absolutely.

In fact, time is our biggest enemy, as well as numbers, sheer numbers.

NICK SCHIFRIN: The majority of remains are considered unrecoverable, but that still leaves tens of thousands that can be recovered, bringing dignity to those who made the ultimate sacrifice.

KELLY MCKEAGUE: It's a covenant made with the service member, fulfilled to the family, but also fulfilled to the nation.

It sends a strong signal.

We will fulfill that ethos of never leaving a fallen comrade behind.

It strikes a chord when a hometown son or daughter come home.

NICK SCHIFRIN: And so it was for Oregon, Ohio, and Uncle Jack and his family the day before Memorial Day.

Does it mean something even more special, the fact that you're doing this on Memorial Day weekend?

SHAWNELLE JOHNS: Yes.

I wanted to make sure that -- not only that we would never forget our own uncle.

I wanted to make sure that everyone out there remembers them all for Memorial Day, because they sacrificed their lives for us and for this country.

And we need to remember them and thank them.

NICK SCHIFRIN: For the "PBS NewsHour," I'm Nick Schifrin.