As a result, some work at the lab has been affected, and security measures have been increased.
No criminal charges have been filed in connection with the case, which is being handled by the FBI. A search warrant executed by the agency has been sealed by a judge.
The lab's director, Michael Anastasio, said a former worker is being investigated, but neither the lab nor the FBI would identify the employee.
"While I cannot discuss the details of what we know and don't know about what happened, I can confirm that classified material was found in her residence," Anastasio told lab employees Thursday.
The Washington Post reported Thursday that the former employee had a high-level security clearance.
"This is a very serious matter for all of us," Anastasio said. "We are taking and must continue to take immediate and appropriate action."
Anastasio did not elaborate on the case but said the FBI interviewed several employees at the lab Wednesday. A lab spokesman declined to comment...