North Korea’s return of what it claims are the remains of U.S. troops killed in the Korean War is the first step in helping bereaved families finally end decades of uncertainty, Defense Secretary James Mattis said on Friday.

“We have families that when they got the telegram have never had closure,” Mattis told reporters. “They’ve never had the body returned. So what we’re seeing here is an opportunity to give those families closure and to make certain that we continue to look for those remaining.”

Earlier on Friday, an Air Force C-17 cargo plane from Hawaii loaded 55 boxes of remains at Wonsan, North Korea, and then flew them to Osan Air Base in South Korea. The repatriation of remains happened on the 65th anniversary of the armistice that suspended fighting on the Korean peninsula. North and South Korea technically remain in a state of war.

“By the way, you noticed there was a U.N. [United Nations] blue flag on each of the boxes,” Mattis said at Friday’s press briefing. “Many of the U.N. nations with us also have missing. We don’t know who’s in those boxes. As we discover it, they’ll be returned. They could go to Australia – they have missing; France has missing. There is a whole lot of us. So this is an international effort to bring closure to those families.”

Before the remains can be sent to their final resting places, U.S. military officials in both South Korea and Hawaii will examine them and conduct forensic analyses to determine if they belong to fallen U.S. service members or others – such as North Korean troops, he said.

“We have no indications that there’s anything amiss, but we don’t know – we can’t confirm that one way or another,” Mattis said. “That’s why we go through all of the forensics.”

The Veterans of Foreign Wars are urging families of U.S. troops listed as Missing in Action in Korea to provide the Pentagon with DNA samples to help accelerate the process of identifying remains, the group announced on Friday.

“Some 5,300 of 7,699 American unaccounted-for war dead are believed to be in North Korea, and 111 of our 126 Cold War missing are in the vicinity of the Korean peninsula, yet family reference samples on file only account for 91 percent of Korean War missing and 85 percent of Cold War losses,” VFW National Commander Vincent Lawrence said in a news release.

Relatives can contact a military service casualty officer to learn how they can provide a DNA sample, the news release says.

The United States is interested in sending teams to North Korea to search for the remains of U.S. service members for the first time since 2005, but that would need to be worked out with the North Koreans, Mattis said.

“We look at it as the first step in a restarted process, so we do want to explore additional efforts to bring others home, perhaps have our own teams go in,” Mattis said. “They’ve been in there before, by the way. So we’re looking at all this, but this is a first step.”

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