Some Ukrainians were without power for hours last month after hackers infiltrated the power grid and were able to turn off the lights, according to a report in The Washington Post.

An official with the cybersecurity company iSIGHT Partners told the newspaper that the Dec. 23 cyber attack appeared to be the first documented time that hackers successfully shut off power.

The official told The Washington Post that the group believed to be responsible for turning off the lights was Russian, and had at one point tried to attack targets in the United States and Europe. But another cyber security expert told the paper it could be difficult to determine the exact circumstances about the breach, including if the alleged hackers were even responsible.