Hillary Clinton has pulled out as keynote speaker for a cybersecurity conference, with organizers saying in an email she couldn't participate due to an "unforeseen circumstance."

The former secretary of state and 2016 Democratic presidential nominee was to have attended the FireEye Cyber Defense Summit, being held on Oct. 9-10, where she was to have engaged in a question-and-answer session with FireEye CEO Kevin Mandia about the "geopolitical landscape and its implications for global cybersecurity today," reports The Daily Caller, citing an email shared on Twitter by federal whistleblower Nate Cain.

FireEye, nor Clinton spokesman responded to a request from The Daily Caller for further information, but a source close to FireEye said "something is up" with Clinton's speech. Her name also is not included in an online list of the summit's speakers.

Clinton came under investigation for running a homemade email server for official business while she was secretary of state.

The server was unveiled after confident Sidney Blumenthal had his AOL account hacked, which revealed emails to her address.

Further, emails from the account of campaign chairman John Podesta were hacked and put out on WikiLeaks.