WASHINGTON (Reuters) – The U.S. Department of Homeland Security said on Tuesday it was helping Ukraine investigate an apparent attack last month on the country’s power grid that caused a blackout for 80,000 customers.

Experts have widely described the Dec. 23 incident at western Ukraine’s Prykarpattyaoblenergo utility as the first known power outage caused by a cyber attack. Ukraine’s SBU state security service has blamed Russia for the incident, while U.S. cyber firm iSight Partners linked it to a Russian hacking group known as “Sandworm.”

In an advisory, DHS said they had linked the blackout to malicious code detected in 2014 within industrial control systems used to operate U.S. critical infrastructure. There was no known successful disruption to the U.S. grid, however.

DHS said the “BlackEnergy Malware” appears to have infected Ukraine’s systems with a spear phishing attack via a corrupted Microsoft Word attachment.

The DHS bulletin from the agency’s Industrial Control Systems Cyber Emergency Response Team, or ICS-CERT, is the first public comment about the Ukraine incident.

A report released by Washington-based SANS Inc over the weekend concluded hackers likely caused Ukraine’s six-hour outage by remotely switching breakers in a way that cut power, after installing malware that prevented technicians from detecting the intrusion. The attackers are also believed to have spammed the Ukraine utility’s customer-service center with phone calls in order to prevent real customers from communicating about their downed power.

DHS and the FBI did not immediately respond to requests for additional comment.

(Reporting by Dustin Volz and Jim Finkle; Editing by Doina Chiacu and Andrew Hay)