Clinton and her top aides had access to a Pentagon-run classified network that goes up to the Secret level, as well as a separate system used for Top Secret communications.

The two systems — the Secret Internet Protocol Router Network (SIPRNet) and the Joint Worldwide Intelligence Communications System (JWICS) — are not connected to the unclassified system, known as the Non-Classified Internet Protocol Router Network (NIPRNet). You cannot email from one system to the other, though you can use NIPRNet to send ­emails outside the government.

Somehow, highly classified information from SIPRNet, as well as even the super-secure JWICS, jumped from those closed systems to the open system and turned up in at least 1,340 of Clinton's home emails — including several the CIA earlier this month flagged as containing ultra-secret Sensitive Compartmented Information and Special Access Programs, a subset of SCI.