North Korean intelligence agencies appear to have built an impressive database of people with expertise in Russian designed liquid-fueled rockets from the Cold War era. This explains the number of former Soviet rocket experts who have come to North Korea to act as highly paid consultants to the North Korean ballistic missile program. North Korean agents have also been found seeking out such experts, or archives containing their knowledge in Russia and nations that used to be part of the Soviet Union.
When the Soviet Union collapsed in 1991, the dissolution deal left equipment and other material (like archives and databases) where it was and belonging to the new country. In the 1990s, deals were made (with Russian negotiators and American cash) to get nuclear and chemical weapons removed and destroyed. But a lot of old archives and retired or suddenly unemployed ballistic missile scientists and technicians were left where they were. This data and these people became a resource for outlaw states (like Iran and North Korea) seeking to build ICBMs. North Korea, and to a lesser extent Iran, have been mining this resource ever since the 1990s. It’s all been done very discretely but the U.S. has become aware of how important these Soviet era resources have been to North Korea and are trying to cut off access to the people and archives.