The Energy Department and National Nuclear Security Administration, which maintains the U.S. nuclear weapons stockpile, have evidence that hackers accessed their networks as part of an extensive espionage operation that has affected at least half a dozen federal agencies, officials directly familiar with the matter said.

On Thursday, DOE and NNSA officials began coordinating notifications about the breach to their congressional oversight bodies after being briefed by Rocky Campione, the chief information officer at DOE.

They found suspicious activity in networks belonging to the Federal Energy Regulatory Commission (FERC), Sandia and Los Alamos national laboratories in New Mexico and Washington, the Office of Secure Transportation and the Richland Field Office of the DOE. The hackers have been able to do more damage at FERC than the other agencies, the officials said, but did not elaborate.