Experts say they have established the world’s first known case of a cyber attack on a power grid, which cut power to more than 600,000 homes in Ukraine in late December. US intelligence agencies and cyber security experts are looking to Russia as the likely source of the attack.

Prykarpattyaoblenergo, an energy company in the Ivano-Frankisvk region of western Ukraine, said on Dec. 23 that a blackout in a large part of the area where it delivers electricity was caused by an “interference” in its systems.

Ukraine’s security service and government blamed Russia for the attack, and launched an investigation. According to Shane Harris at The Daily Beast, experts at the CIA, National Security Agency and the Department of Homeland Security are also investigating whether samples of malware recovered from the company’s network indicate that the blackout was caused by hacking and whether it can be traced back to Russia. If confirmed, it would be the first international cyber attack to cause a power outage.

On Monday, Jan. 4, researchers from the global cyber security company iSIGHT and antivirus firm ESET claimed they had samples of the malicious code that affected three of the region’s power companies, causing “destructive events,” Ars Technica reported.

It’s a scenario that has long worried cyber security experts. ”It’s a milestone because we’ve definitely seen targeted destructive events against energy before—oil firms, for instance—but never the event which causes the blackout,” John Hultquist, director of cyber espionage analysis at iSIGHT told Ars Technica.

Experts say that the malware they found is a virus called BlackEnergy, which has been used in the past to sabotage companies and news organizations, including in the US. The virus has since been updated, acquiring new malicious capabilities.

Analysts at iSIGHT say the group behind the virus, which they have called “the Sandworm gang,” targeted NATO, Ukraine, Poland and European industries in 2014.