The US government has linked North Korean hackers to the theft of hundreds of millions of dollars’ worth of cryptocurrency tied to the popular online game Axie Infinity, according to digital forensics firms.

Ronin, a blockchain network that lets users transfer crypto in and out of the game, said digital cash worth almost $615 million was stolen on March 23.

No one has explicitly assigned blame for the hack, but on Thursday the US Treasury identified a digital currency address used by the hackers as being under the control of a North Korean hacking group often dubbed “Lazarus.”

Blockchain analytics firms including Chainalysis and Elliptic said the designation confirmed that North Korea was behind the break-in.

The Treasury did not immediately return a message seeking comment. Aleksander Larsen, the co-founder of Sky Mavis, which makes Axie, declined comment. CrowdStrike, which was hired by Sky Mavis to investigate the breach, also declined comment.

A person familiar with the matter, however, confirmed that North Korean hackers had been the focus of the cybersecurity firm’s investigation for the past couple of weeks.