The FBI seized four servers from the State Department building a few weeks ago as part of the investigation into classified material improperly stored on Hillary Clinton’s secret email server, particularly the Top Secret documents that have been discovered in her email trove.

This revelation comes from Bill Gertz at the Washington Free Beaconciting anonymous sources familiar with the investigation. Gertz notes that the State Department referred questions about the seized servers to the FBI, which declined to comment on the matter.

It would be interesting to know exactly what the FBI computer forensics team is looking for on these servers, and whether they were part of the State Department’s secured or unsecured email networks. Investigators are probably trying to resolve the “air gap” issue: the physical impossibility of forwarding communications from the classified network to an unsecured system, such as Clinton’s home-brew machine.

Security experts have explained that the classified network is not electronically linked with any other system. Someone needed to pull up the Top Secret documents, transfer them to portable media such as a jump drive, and then load them onto an unsecured system to forward them to Clinton. Such portable media devices are not allowed in the safe rooms where Top Secret documents are viewed. Printouts that could be scanned into unsecure systems are not supposed to be removed from such facilities, either.

Gertz quotes Chris Farrell, an investigator for watchdog group Judicial Watch, suggesting the FBI may be acting on information extracted from the jump drives it seized from Clinton’s lawyer, David Kendall:

In the midst of what I believe to be a forensic examination of the hardware that Kendall surrendered on behalf of Mrs. Clinton, any serious national security investigation would seek to track all emails inbound and outbound. If they are doing that tracking of email since she was Secretary of State, then they would be looking at any email that could have crossed into a State server.

Kendall evidently still has some explaining to do, as Gertz notes the State Department contacted him on Tuesday, looking for more work-related emails that Clinton neglected to hand over as part of her legally-compelled document production.

FBI agents physically removing four servers from the State Department seems like rather a big deal, especially if they were active components of either the secured or unsecured mail systems. Few network administrations would cheerfully shrug off the loss of four servers from their operations. It’s possible these machines were retired and essentially sitting in storage before the FBI took them – that seems like a detail State and/or the FBI should clear up in a public statement.