The FBI has seized four State Department computer servers as part of its probe into how classified information was compromised on former Secretary of State Hillary Clinton’s private email system, according to people familiar with the investigation.

The four servers, which were located at the State Department’s headquarters building, were seized by the FBI several weeks ago. They are being checked by technical forensic analysts charged with determining how Top Secret material was sent to Clinton’s private email by State Department aides during her tenure as secretary from 2009 to 2013, said two people familiar with the probe. The people spoke on condition of anonymity because it is an ongoing investigation.

State Department spokesman John Kirby referred questions about the computer servers to the FBI. An FBI spokeswoman, Carol Cratty, declined to comment.

No other details about the servers, including whether they are part of the department’s classified system, or used for unclassified information networks, could be learned.

A spokesman for the Clinton campaign did not respond to an email request for comment.