Back in May, one of the biggest ransomware attacks in history took place: WannaCry. 300,000 devices across almost 150 countries around the world were infected, and while several security agencies, Microsoft, and the UK said North Korea was responsible, the US never publicly attributed the attack to anyone—until now. Unsurprisingly, it also blames Kim Jong-un’s regime.

In an op-ed piece for the Wall Street Journal, Donald Trump’s homeland security advisor, Tom Bossert, wrote: “The attack was widespread and cost billions, and North Korea is directly responsible.”

While the UK had said that it was “all but certain” North Korea was behind the attack, this is the first time the US has pointed the finger of blame. Bossert said those responsible would be held accountable for their actions.

The Trump administration believes “with a very high level of confidence” that the Lazarus Group, the hackers working at the behest of the North Korean government, was behind WannaCry. It was also responsible for the Sony hack in 2014, the $80 million Bangladesh Bank heist in 2016, and recent attacks on workers at Bitcoin companies.

“As we make the internet safer, we will continue to hold accountable those who harm or threaten us, whether they act alone or on behalf of criminal organizations or hostile nations,” wrote Bossert.

“North Korea has acted especially badly, largely unchecked, for more than a decade, and its malicious behavior is growing more egregious. WannaCry was indiscriminately reckless.”

Soon after the WannaCry attack, Simon Choi, a senior researcher with Hauri who has studied North Korea’s hacking program extensively, said he believed it was linked to the hermit nation. "It is similar to North Korea's backdoor malicious codes," he explained, adding that the country has been developing and testing ransomware since August 2016.

Pyongyang has called the links “ridiculous,” and repeatedly denied it had anything to do with WannaCry. It also said that the UK’s earlier accusations were a “wicked attempt” to tighten international sanctions on the country.

The White House is expected to give an official statement blaming North Korea on Tuesday.