Here’s How Hard It Is to Bring Home Remains of U.S. Soldiers, According to Experts

Following through on an agreement made at a June summit between U.S. President Donald Trump and North Korean leader Kim Jong Un in Singapore, North Korea on Friday returned what is believed to be the remains of 55 American servicemen killed during the Korean War.

After retrieving the remains from a seaside city in the North, a U.S. military aircraft returned to Osan Air Base outside of Seoul where U.S. servicemen and a military honor guard stood by for their homecoming, which coincides with the 65th anniversary of the end of the Korean War.

This is the first repatriation of American remains from North Korea since 2005, a promising development for the veteran community and their families. Nearly 7,700 American military service-people are still unaccounted for from the Korean War, according to the Department of Defense POW/MIA Accounting Agency, of which approximately 5,300 are thought to be in North Korea. Pyongyang has long suggested that the remains were in its possession, but retrieval has been thwarted by diplomatic tensions with the communist regime.

“It’s huge development for all of the war families,” says Dr. Timothy P. McMahon, director of the DoD DNA Operations for the Armed Forces Medical Examiner System. “It’s something they have been looking forward to and needing for the last 26 years, if not the last 60 years.”

While the return has been hailed by veterans and their families, only part of the job is done. The remains will still need to undergo forensic analysis to confirm their identities, and thousands more still need to be found. But when bodies have been underground for decades, what exactly does it take to repatriate them? TIME asked the government agency in charge, the DPAA, to find out.

What is the DPAA?

The Defense POW/MIA Accounting Agency is a division within the Department of Defense that is tasked with accounting for Americans lost in conflict, including in World War II, the Vietnam War, Korean War, Cold War, Iraq War, and others. The job of retrieving fallen personnel used to be done by several disparate government departments, but in 2015, former secretary of defense Chuck Hagel brought them under one umbrella. Operational since 2016, the agency has about 700 employees, with research teams divided geographically.

How many servicemen are missing?

More than 82,000 Americans have not returned home between World War II and the Gulf War. The vast majority — 73,000 — were lost in World War II. The DPAA estimates that about 34,000 are recoverable. 7,697 remain missing from the Korean War, 1,597 from Vietnam, and 126 from the Cold War. Of the missing Korean War veterans, 5,300 are believed to be located in North Korea.

How does repatriation work?

The DPAA goes through the same basic procedure for every body: locate, recover, identify.

Location starts in the filing cabinet. Agency historians basically follow a paper trail, piecing together historical documents to draw a circle on the map around a point where a body might be. They then use archives and witness accounts to narrow that circle down. But as time passes, this becomes increasingly difficult, as key witnesses with information age and pass away.

“Time is always the enemy in all of our cases,” says Chuck Prichard, a DPAA spokesman. “Each one of those can take years, even decades.”

Eventually the research develops into an investigative team for a recovery mission. This involves tasks like surveying land and working with local governments to obtain permits for disinterment. One of they key challenges is nature, as land eats away at remains. This is especially problematic in countries like Vietnam where land can be acidic.

If and when remains are recovered, they will then be sent to labs at air force bases in either Hawaii or Nebraska. Using forensic anthropology, odontology, DNA and other scientific methods, anthropologists examine all surviving artifacts, from military uniforms to life support equipment, to establish a biological profile. The biggest obstacles here are lack of information and commingling. Many of the remains turned over by North Korea in the past have arrived with little or no information as to where they were found. And in 2005, among the 208 caskets returned by Pyongyang, the remains of about 400 individuals were found commingled together.

DPAA partners with Armed Forces Medical Examiner System (AFMES), based in Delaware, to conduct DNA testing. With an extensive family reference database, AFMES’ forensics experts compare the sequences they uncover to reference samples on file, which are either from the missing individual or from a family member. Of all the missing personnel from the Korean War, 92% have a DNA reference card.

“The way we think about it is like a jigsaw puzzle,” says Dr. McMahon from AFMES. “We use mitochondrial DNA to establish the border, then we use further discriminatory tests to segregate them into individuals.”

“The biggest obstacle isn’t identifying them,” McMahon adds. “It’s just getting them turned over to us.”

Beyond this general framework, every single case is unique in terms of cost, time and resources. Some cases in more remote areas will require several helicopter trips, while others will call for more diggers.

How does repatriation work with North Korea?

Diplomatic tensions have consistently stymied repatriation efforts with Pyongyang. Following the armistice in 1953, which established the demilitarized zone (DMZ) between the two Koreas, both sides were required to hand over the dead. The U.S. received an estimated 4,219 human remains, of whom 2,944 are known or believed to be Americans.

Between 1996 and 2005, an agreement between the U.S. and North Korea allowed for recovery of some 229 personnel. DoD entities that preceded the formation of DPAA worked jointly with the Korean People’s Army and the Ministry of Foreign Affairs in 33 missions over that period. But President George W. Bush ended that agreement because of North Korea’s development of nuclear weapons and safety issues surrounding the recovery teams.

A Defense Department report later found that during that recovery period, North Korea had already recovered bones and planted them back in fighting positions, according to the New York Times.

Since 2005, no recovery teams have been allowed to cross into the country, and it’s unclear whether that will change following the Singapore summit.