The nosedive in cryptocurrency markets has wiped out millions of dollars in funds stolen by North Korean hackers, four digital investigators say, threatening a key source of funding for the sanctions-stricken country and its weapons programs.

North Korea has poured resources into stealing cryptocurrencies in recent years, making it a potent hacking threat and leading to one of the largest cryptocurrency heists on record in March, in which almost $615 million was stolen, according to the U.S. Treasury.

The sudden plunge in crypto values, which started in May amid a broader economic slowdown, complicates Pyongyang's ability to cash in on that and other heists, and may affect how it plans to fund its weapons programs, two South Korean government sources said. The sources declined to be named because of the sensitivity of the matter.