Recent photos suggested the North Koreans were in the process of dismantling a test engine stand at a satellite launching station, but various analysts warn the gesture is mostly symbolic since tests there were already complete.
North Korean leader Kim Jong-un also vowed during the June 12 summit with US President Donald Trump in Singapore to work toward denuclearization, but there has not been a concrete agreement to accomplish that goal.
He also confirmed a single military dog tag was provided with the remains and that the family of that soldier has been notified. He said that when an individual is identified, those remains will be returned.
Together, Pompeo's testimony and the new assessment confirm that North Korea continues to not only produce ballistic missiles capable of threatening the United States, but also fissile material to create new warheads to sit atop those missiles.
But they did not agree on details, which will be further discussed through working-level talks, the ministry said.
No additional information was given to help US forensics experts determine who is in those boxes, according to the official.
But Narang said the most interesting revelation was not that the North was continuing to build its nuclear forces, but rather the apparent strategy of deception. The North Koreans collected the remains, and US investigators will have to do the examination without the forensic information they normally would have, such as the approximate place of the burial and the conditions around it.
Most died on the battlefield and were buried in shallow graves or in cemeteries that were meant to be temporary, but some also perished in POW camps run by North Korea or China, experts say.
Defense Secretary Jim Mattis has welcomed the return of the 55 boxes but acknowledged it's not certain who is inside.
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Members of support group the Korean War Prisoners Of War and Missing In Action Network have told families to be cautious about the latest releases, saying they could be from other nationalities, not necessarily American.
But John Byrd, director of scientific analysis at the Defense POW/MIA Accounting Agency (DPAA), said preliminary findings suggested that "they are likely to be American remains".
Immediately after meeting with Kim, Trump assured the world that "there is no longer a Nuclear Threat from North Korea."
United Nations honor guards carry a casket containing remains transferred by North Korea onto a plane at Osan Air Base in Pyeongtaek, South Korea, on Wednesday.
The remains were first flown from the North Korean city of Wonsan last Friday, on the 65th anniversary of the armistice, and were greeted at Osan Air Base by hundreds of US service personnel and their families.
The lab will then set about trying to match any American remains with the identities of MIA personnel.
Narang said the aim of such a strategy would be to obtain sanctions relief, while still retaining a deterrent - what he called "the best of all worlds for North Korea".
The U.S. -led United Nations Command planned a formal repatriation ceremony Wednesday at Osan, where the remains have been examined and catalogued by DPAA.
USA officials have for years warned that Pyongyang was developing nuclear warheads small enough to fit atop an ICBM capable of reaching the US mainland.