Hillary Clinton's private emails may have contained information classified above "top secret."

A Jan. 14 letter from Charles McCullough, inspector general for the intelligence community, indicated "several dozen" private emails contained information ranging from "confidential" to "special access programs," or SAP, a unique kind of intelligence that is considered more sensitive than top secret.

Access to SAP is highly restricted and fiercely protected by the intelligence community.

Officials from an unnamed intelligence agency must now work with the State Department to decide how the SAP emails will be handled in an open Freedom of Information Act lawsuit that will require State to publish all of Clinton's emails by Jan. 29.

McCullough sent the letter to lawmakers last week after receiving a sworn declaration from intelligence officials involved in the process of screening emails for release through the high-profile FOIA case. One official told NBC News that McCullough and members of his staff had to receive a higher security clearance just to read the declaration given its high level of classification.

The discovery of such sensitive material on Clinton's private server may heighten scrutiny in an already high-profile security probe of the personal network Clinton established while serving as secretary of state.

McCullough said his office's recomendation to the State Department that the emails be handled on a secret-level server, given the presence of information classified at that level and above, "remains unresolved."

While the letter was addressed to the chairmen of both the Senate Intelligence Committee and the Foreign Relations Committee, several other congressional committees received a copy, as did the State Department inspector general and the Office of the Director of National Intelligence.