An energy company providing power in several western U.S. states experienced a “denial-of-service condition” serious enough to warrant reporting it to the government’s energy authority.

The “cyber event” resulted in “interruptions of electrical system operations” for more than 10 hours on March 5, according to an electric emergency and disturbance report filed with the Department of Energy by the affected company.

E&E News first reported the “cyber event” last week. It was later reported as a denial-of-service condition, in which systems are knocked offline.

“DOE received a report about a denial-of-service condition that occurred at an electric utility on March 5, 2019 related to a known vulnerability that required a previously published software update to mitigate,” a spokesperson for the Department of Energy told TechCrunch. “DOE continues to work with our industry partners through the ISACs to ensure the dissemination of the appropriate mitigation information to manage their associated risks.”

The energy company wasn’t named, but it provides power and energy to customers across Los Angeles in California, Salt Lake County in Utah, and Converse County in Wyoming.

“The incident did not impact generation, the reliability of the grid or cause any customer outages,” said the DOE spokesperson.

Western Electricity Coordinating Council, the regional reliability authority for the affected area, did not immediately comment.

Power networks — considered critical infrastructure by the U.S. government — have long been a target for hackers, but successful attacks are rare. Russian hackers were blamed for a power outage in Ukraine at Christmas in 2015, leaving a quarter-of-a-million residents without power for two days. Venezuela recently blamed a cyberattack for a power outage earlier this year, but the claims remain unfounded.

The Trump administration also blamed Russia for attempting to break in to U.S. power grids.

Updated headline to clarify the denial-of-service condition, not attack.