In a surprise press conference, Comey said the agency's investigation found that Clinton did send and receive classified information on her private email system but noted that there was no evidence that she deliberately attempted to mislead investigators.

The FBI director said Clinton's conduct did not meet the threshold used to prosecute past violators who shared classified information over unclassified channels.

"In looking back at our investigations into mishandling or removal of classified information, we cannot find a case that would support bringing criminal charges on these facts," Comey said.

He continued: "All the cases prosecuted involved some combination of clearly intentional and willful mishandling of classified information, or vast quantities of materials exposed in such a way as to support an inference of intentional misconduct, or indications of disloyalty to the United States, or efforts to obstruct justice. We do not see those things here."