HONOLULU — A Christmas card arrives every December in Bill Belcher’s mailbox, sent by the daughter of a 1940s fighter pilot he unearthed in the mountains of Papua New Guinea.

The excavation was 20 years ago. But the woman’s gratitude for her father’s repatriated remains hasn’t diminished with time.

Nor has Belcher’s focus. The son of a Korean War and Vietnam War veteran, he manages a team of forensic anthropologists and dentists, medical examiners and historians with a daunting mission — to find and identify U.S. service members from conflicts dating to World War II.

“I grew up with the idea that we might see that black car come in and tell us that Dad was gone,” said Belcher, who is deputy director of the Defense POW/MIA Accounting Agency’s $80 million forensic laboratory here. “And so, in the back of my mind, whenever I’m doing this work, I think, ‘This could be my father.’ ”

The lab and its mission are again in the spotlight. In an unusual collision of geopolitics and humanitarianism, North Korea recently handed over what could be the remains of more than 50 American service members still missing from the Korean War. The transfer was the first in more than a decade, and DPAA Director Kelly McKeague called it a “confidence building, relationship-enhancing overture” that could someday allow the agency to resume its recovery efforts in that country.

For now, the most difficult work is taking place on 70 exam tables in the sterile, harshly lighted facility at Joint Base Pearl Harbor-Hickam, where staffers probe and test fragments of past lives. They puzzle over the abstract clues, trying to match a tooth filling with a missing soldier’s dental record or a clavicle bone with the chest X-ray of a serviceman who once had a tuberculosis screening.

Even at the world’s preeminent forensic anthropology facility, progress is slow; an individual case can take from weeks to a decade. This year, 162 servicemen have been identified. But nearly 1,400 cases are pending — with more than 500 others listed as inactive because the clues ran out before an identification could be made.

“I think there’s a disjuncture between popular-cultural expectations and what the science can do and how fast it can do it,” said Sarah Wagner, a social anthropologist at George Washington University who is researching how the U.S. government tries to account for missing service members. “These are not bones from a fresh body like you see on TV shows. They are degraded and don’t easily yield up DNA.”

The remains of Vietnam War-era pilots who died after being shot down, for example, may be charred so badly that no genetic material can be extracted. Soldiers buried on the battlefield tend to be fairly well preserved.

The lab’s tradition dictates that bones be sorted, measured and displayed on tables in the shape of a skeleton, head positioned toward the room’s perimeter. If these bones could jolt to life, or so goes the thinking, the first thing they would see upon sitting up is the American flag hanging at the center.

“Every one of these people we’re working on is a hero — I truly believe that,” Belcher said as he walked through the three-year-old lab, where every bone is assigned an identification number until it gets matched to a name. “Nobody really takes priority above anybody else.”

His 92-person team is expanding the forensic use of a technology traditionally used to spot counterfeit Kobe beef. With isotope analysis, lab technicians seek to match the unique quantity of elements such as carbon, oxygen or strontium in a human bone to the quantity of these same elements in the bedrock of landscapes around the world.

Bones take on the isotopic signature of the place where a person was raised. On digital maps of the United States, staff members plot the hometowns of missing service members based on the isotopic signatures shared by their early-childhood geography and their bones.

Ten years ago, this technology could differentiate the bones of a native-born soldier from those of an immigrant. Nowadays it can pinpoint a service member’s origin down to a specific area — a particular Hawaiian island, perhaps, or a corner of the Plains.

The pillars of the lab’s effort rest on simpler techniques, however. The shape and internal structure of a bone is crucial in determining age and ethnicity, as well as differentiating the remains of humans and animals.

As clues are revealed, such as a probable shoe size or a nasal sill indicating a large nose, lab technicians mine the personnel files of more than 82,000 missing troops in an attempt to narrow their search. The agency estimates that 41 percent of missing U.S. service members are recoverable. Most of the others are presumed lost in the ocean’s depths.

“I think it defines us as a nation that here we are decades later, still searching, still finding, still recovering and still identifying those that made the ultimate sacrifice,” McKeague, a retired Air Force major general, said late last month, quietly talking about the challenges in his office at the DPAA.

Case statuses are fluid. Recent DNA advances have allowed for the testing of bones as small as a pinkie-finger joint — and the reopening of inactive cases in which the recovered fragments were previously too meager to be useful.

That analysis is conducted 4,900 miles away at the Armed Forces DNA Identification Laboratory in Dover, Del., which also collects DNA from missing service members’ families. For the 7,700 Korean War troops never accounted for, the lab has family samples for 92 percent of them. But for World War II, the same is available for 4 percent of the 72,000 soldiers, sailors, airmen and Marines yet to be located.

Research underway in Dover could soon broaden possible DNA matching to a family’s fourth generation — an important advance, given the passage of time.

“Time is our biggest enemy,” McKeague said. “Families are passing away. Witnesses are dying. Urbanization is affecting recovery sites. But we can’t just do this willy-nilly. I always tell the families that the United States is still committed to doing this in steadfast, inordinately expansive ways, and that should give them hope.”

Between 1996 and 2005, joint missions of the U.S. and North Korean governments recovered 430 service members. Seventy remain unidentified.

The latest repatriation of remains brings Christina Brown of Saratoga Springs, N.Y., a small promise of hope. She was a few months old when her father, Army 1st Lt. Norman Melander, vanished after completing a successful attack in Korea’s heavily forested Chorwon region.

His loss remains excruciating. Even 67 years later she can barely face it, let alone talk about it. Brown defers to loved ones to answer questions.

“She never knew her father,” said Brown’s longtime boyfriend, Leonard Galasso. “That’s a big emptiness in her life. And she still can’t deal with it.”

He continued: “If there was a miracle and her father’s remains are one of the 55, that would be some closure for her. Because right now, it’s like he’s in the abyss.”