North Korea success in testing an intercontinental ballistic missile that appears able to reach the United States was possible through black-market purchases of powerful rocket engines probably from a Ukrainian factory with historical ties to Russia’s missile program, according to an expert analysis being published this week and classified assessments by American intelligence agencies, according to NYT newspaper.

The studies may solve the mystery of how North Korea began succeeding so suddenly after a string of fiery missile failures, some of which may have been caused by American sabotage of its supply chains and cyberattacks on its launches.

After those failures, the  North Koreans changed designs and suppliers in the past two years, according to research by Michael Elleman, a missile expert at the International Institute for Strategic Studies.