(CNN) -- Two years ago, U.S. military officials came knocking on Michael Frisbie's door, asking for information on his family tree.
They returned about a year ago, this time informing him that the remains of his great-great-uncle -- a soldier missing in action since World War I -- had been identified.
"It was overwhelming," said Frisbie's wife, Leanne. "They were just looking through the family tree to make sure that they had the right family, and bingo, they found us."
Frisbie, 43, says he had no clue that Army Pvt. Thomas D. Costello even existed. Frisbie's parents divorced when he was only 6 months old, and he never got to know his paternal relatives.
"I can't believe they went to all this extent to find me, which is good, though, because I want to honor the soldier," said Frisbie, who lives in Stockton Springs, Maine.
More than 90 years after his death, Costello was finally buried with full military honors Monday.
Costello, from New York, enlisted in the Army on September 19, 1917, and was part of the 60th Infantry Regiment, 1st Brigade, 5th Infantry Division, according to military documents.
On September 16, 1918, with World War I nearing an end, Costello and his fellow troops encountered heavy artillery and machine-gun fire near Jaulny, in northern France. He died of a shrapnel head wound, Frisbie said.
Costello's fellow troops buried him with two other soldiers in a wooded area between Bois de Bonvaux and Bois de Grand Fontaine. Based on enlistment records, he was estimated to be 26 when he died.
Despite efforts by his sister and Army officials to find and retrieve Costello's remains, the grave could not be found. Costello was not married and did not have children.
In September 2006, people hunting for metal in the area found human remains and World War I artifacts, U.S. Army officials said.
A Joint POW/MIA Accounting Command team, operating near the location, was notified of the discovery and recovered human remains upon excavating the site, about 20 miles from the coordinates Costello's commander gave when the war ended.
Frisbie said buttons, gloves and boots were recovered at the site, which appeared to be at the edge of a field with overgrown trees, judging from photos given to him by the military.
"They found some rosary beads, which we now have," he said, adding that because Costello's family was known to be Catholic, it was the one item that probably belonged to the fallen soldier.
Scientists from the POW/MIA command laboratory used dental comparisons as well as other forensic identification tools and circumstantial evidence in the identification of the remains.
The tedious search by genealogists for relatives of unaccounted fallen soldiers is only part of the work done by an arm of the U.S. Defense Department led by the Defense Prisoner of War/Missing Personnel Office.
Together with its operational units, 600 personnel -- including forensic anthropologists, DNA scientists, archaeologists and explosive ordnance specialists -- work to locate, recover and identify remains and return them to family members. Many of the personnel come from military backgrounds, said Larry Greer, public affairs director for the personnel office.
Once all the information about the remains is verified, the process of recovering a missing soldier is detailed in a book that is given to family members. It's through such a book that Frisbie was able to learn much about his uncle.
"You can think of [each case] as a big-city police detective case; however, our cases are at a minimum 40 years old. And some of them are 60, 70 years old," Greer said.
World War I finds are rare, though, and the Department of Defense has identified only five U.S. soldiers from the "great war" since 2006, Greer said. That leaves more than 3,000 U.S. troops missing and unaccounted for in that war.
It's a small proportion of the 80,000 still missing from other wars that have ended: World War II, the Korean War, the Cold War and the Vietnam War.
All the service members listed as POW/MIA in the 1991 Gulf War have been accounted for, according to the personnel office's website.
"We have all the wartime records of everybody who is missing. Every day of the year, our analysts build case files," Greer said.
"If a family member were to write us and say, 'Please, my loved one is missing; please go find him,' we would respond to that family member and say, 'Well, let us show you what we've already done on his case.' And oftentimes, they're quite surprised to learn of it."
About $105 million is allocated annually to recovering missing soldiers from past wars, a reflection of the military's commitment to "leave no man behind," Greer said.
Part of that pays for airfare for relatives such as Frisbie, his wife and daughter, who attended Costello's funeral at Arlington National Cemetery on Monday.
Soldiers from the Army's 3rd U.S. Infantry Regiment, the Old Guard, conducted the ceremony. Costello was buried in section 34 with military honors, including a casket team, a rifle salute and a bugler who sounded taps.
As the ceremony ended, Brig. Gen. Donald Rutherford handed the flag to Frisbie.
Lt. Col. Judy Law of the Defense Prisoner of War/Missing Personnel Office and Col. Brice Houdet, French military attaché, were present and offered their condolences to the relatives.
"I think it's a great thing," Frisbie said of the office's work. "Any man who gave his life for the country deserves this."