FBI found that Clinton exchanged emails with people whose email was compromised by foreign sources

Bill Gertz:

The FBI’s once-secret report on former Secretary of State Hillary Clinton’s email system reveals that messages sent to aides were compromised by hostile foreign actors.

The FBI did not elaborate on the identity of foreign cyberattacks that normally refer to activities of sophisticated foreign intelligence services or criminal groups. The bureau was also unable to “conclusively” say whether classified information sent on Mrs. Clinton’s emails and mobile phones was compromised by “cyber means,” the report said. FBI cybersleuths blamed their inability to obtain all Mrs. Clinton’s old devices and computers for the inconclusive results.

However, the FBI did conclude that Mrs. Clinton sent emails to people with computer systems that were penetrated by foreign hackers.

“The FBI did find that hostile foreign actors gained access to the personal email accounts of individuals with whom Clinton was in regular contact, and, in doing so, obtained emails sent to or received by Clinton on her personal account,” the report noted.
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This may be a reference to the Guccifer hack of Sidney Blumenthal's email account.  Some of his emails with Hillary Clinton surfaced in the course of the Benghazi investigation.