A sophisticated hacking group recently attacked a U.S. public utility and compromised its control system network, but there was no evidence that the utility’s operations were affected, according to the Department of Homeland Security.

DHS did not identify the utility in a report that was issued this week by the agency’s Industrial Control Systems Cyber Emergency Response Team, or ICS-CERT.

“While unauthorized access was identified, ICS-CERT was able to work with the affected entity to put in place mitigation strategies and ensure the security of their control systems before there was any impact to operations,” a DHS official told Reuters on Tuesday.

Such cyber attacks are rarely disclosed by ICS-CERT, which typically keeps details about its investigations secret to encourage businesses to share information with the government. Companies are often reluctant to go public about attacks to avoid potentially negative publicity.