Hillary Clinton's personal server system was almost certainly hacked by at least five foreign intelligence agencies, according to a published report.

Speaking with Fox News, two unnamed "sources with intimate knowledge of the FBI investigations" said there was a "99 percent" probability Clinton's devices had been hacked. It was unclear which countries were involved, but it was previously known Clinton had been targeted by intelligence agencies in China, Germany, South Korea and Russia.

Between October 2013 and December 2014 alone, a firewall maintained by private security firm SECNAP detected 53 possible attempts to breach Clinton's server. The server did not have a firewall for several months in mid-2013.

It is unclear if or when breaches may have taken place. Clinton used the server setup to store classified information from the time she assumed office as secretary of state in 2009 until her files were finally destroyed in the first half of 2015.