WASHINGTON (AP) — The FBI said this week that it is investigating “suspicious activities” on an internal system that contains sensitive information related to surveillance operations and investigations.

The bureau is working to determine the scope and impact of the problem, according to a notification sent to members of Congress that says the unnamed culprit is using sophisticated techniques to exploit FBI network security controls.

The notification, obtained Thursday by The Associated Press, says that the FBI on Feb. 17 began investigating abnormal log information related to a system on its network.

“The affected system is unclassified and contains law enforcement sensitive information, including returns from legal process, such as pen register and trap and trace surveillance returns, and personally identifiable information pertaining to subjects of FBI investigations,” said the notification reviewed by The Associated Press.

A pen register is a common surveillance tool that enables law enforcement to log phone numbers dialed by a particular line.

The FBI confirmed the cyber incident in a statement but did not provide additional details.

“The FBI identified and addressed suspicious activities on FBI networks, and we have leveraged all technical capabilities to respond,” the FBI said. “We have nothing additional to respond.”

Neither the FBI statement nor the notification identified who might be responsible for the incident, but the bureau and other federal agencies have long been targets of foreign hackers seeking to spy on sensitive operations and decision-making.

In this instance, the FBI said, the techniques being used were “sophisticated” and included leveraging a commercial internet service provider vendor’s infrastructure to exploit FBI network security controls.