In a secret letter to President Trump in December 2018, North Korean leader Kim Jong Un likened the two leaders’ budding friendship to a Hollywood romance. Future meetings with “Your Excellency,” Kim wrote to Trump, would be “reminiscent of a scene from a fantasy film.”

Yet even as he penned the words, Kim was busy creating an illusion of a different kind. At six of the country’s missile bases, trucks hauled rock from underground construction sites as workers dug a maze of new tunnels and bunkers, allowing North Korea to move weapons around like peas in a shell game. Southeast of the capital, meanwhile, new buildings sprouted across an industrial complex that was processing uranium for as many as 15 new bombs, according to current and former U.S. and South Korean officials, as well as a report by a United Nations panel of experts.

The new work reflects a continuation of a pattern observed by analysts since the first summit between Trump and Kim in 2018. While North Korea has refrained from carrying out provocative tests of its most advanced weapon systems, it never stopped working on them, U.S. intelligence officials said.