The US government is officially blaming North Korea for the WannaCry ransomware attack that struck computers across the world back in May.
Tom Bossert, President Donald Trump's homeland security adviser, made the claim in an op-ed to the Wall Street Journal on Monday.
"The attack was widespread and cost billions, and North Korea is directly responsible," Bossert wrote.
"We do not make this allegation lightly," he added. "It is based on evidence. We are not alone with our findings, either."
Other governments and private companies including the UK and Microsoft agree with the US's conclusion, he said. What evidence they have wasn't presented in the op-ed, but the White House reportedly intends to issue a follow-up statement on Tuesday.
Bossert also warned that North Korea's "malicious behavior" has been ramping up, but that the US government intends to respond.
"Malicious hackers belong in prison, and totalitarian governments should pay a price for their actions," he wrote, adding "The tool kits of totalitarian regimes are too threatening to ignore."
The op-ed called on world governments and businesses to cooperate with the US to fight cyber threats. But it added: "When we must, the US will act alone to impost costs and consequences for cyber malfeasance."
Bossert didn't say what specific actions the US might take against North Korea, but he said it was important to call out the bad behavior. He also noted that Microsoft did make efforts to disrupt activities by North Korean hackers last week, but Bossert didn't elaborate.
Microsoft did not immediately respond to a request for comment.
WannaCry was a massive cyber attack that infected vulnerable Windows machines with ransomware, locking out users from their computers. What made WannaCry particularly devastating was that it managed to spread itself like a computer worm to over 200,000 machines through the use of stolen NSA cyberweapons.
Days after the assault, security researchers began to suspect North Korea was behind the attack. An earlier version of the WannaCry ransomware shared coding similarities with past hacks from the Lazarus Group, a shadowy hacking collective many believe work for the North Korean government. North Korea has reportedly denied any involvement with WannaCry.
Three years ago, the Obama administration blamed North Korea for the hack of Sony Pictures, and later hit the country with sanctions.