North Korea was behind the cyberattack that infected over 200,000 computers in over 150 countries this past summer, the Trump administration revealed Monday.

“After careful investigation, the U.S. today publicly attributes the massive ‘WannaCry’ cyberattack to North Korea,” Thomas Bossert, the president’s homeland security adviser, wrote in a Wall Street Journal op-ed Monday evening. “The attack was widespread and cost billions, and North Korea is directly responsible.

“We do not make this allegation lightly. It is based on evidence. We are not alone with our findings, either,” he explained.

The mid-May attack infected hundreds of thousands of computers, particularly those using older Windows operating systems, with ransomware, locking computers and demanding payments of $300 in the form of Bitcoin, a kind of cryptocurrency. The cyberattack hit Europe and Asia hard, knocking one-fifth of all hospitals in the U.K. offline.

The attack on the healthcare system potentially put lives at risk.

The allegations against North Korea are not surprising, as the rogue regime has long been the primary suspect. Not long after the attack, cybersecurity experts detected links to other hacks carried out by the North, such as the Sony Pictures hack. Others were able to connect the attack to the Lazarus Group, a team of hackers with ties to North Korea.

The National Security Agency linked North Korea to the worm’s creation in June. The British government accused the North in October, and the CIA made a similar assessment the following month, according to The Washington Post.

North Korea has been developing an elite cyberwarfare division for years, as cyberattacks are cheap, asymmetric warfare options for the rogue regime.

“While I would not characterize them as the best in the world, they are among the best in the world and the best organized,” Army General Vincent Brooks, commander of U.S. Forces Korea, said of North Korea’s cyberwarfare capabilities last year.

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