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U.S. Democratic presidential candidate Hillary Clinton speaks during a campaign rally at Iowa State University in Ames, Iowa, Jan. 12, 2016. Reuters

Emails found on Democratic candidate and former Secretary of State Hillary Clinton’s personal server contained information classified at levels higher than previously known, a document obtained by NBC News revealed. Some of Clinton’s emails reportedly contained information classified as Top Secret/Special Access Program, which is a secrecy label that includes some of the most closely held U.S. intelligence matters, according to a letter to lawmakers from the intelligence community’s internal watchdog.

American intelligence officials reportedly told NBC News that the emails are not the same as two from Clinton’s servers that have long been reported as containing top-secret information. The letter to lawmakers did not make clear whether Clinton sent or received the emails.

The letter, sent Jan. 14 from Intelligence Community Inspector General Charles McCullough to senior lawmakers, laid out the findings of a recent comprehensive review by intelligence agencies that identified "several dozen" classified emails including a specific intelligence known as "special access programs" (SAP), according to Fox News. Access to SAP is reportedly restricted to those on a “need-to-know” basis because exposure of the intelligence would likely reveal the source.

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Democratic U.S. presidential candidate Hillary Clinton speaks during "King Day at the Dome 2016" ceremonies in front of the South Carolina State House in Columbia, Jan. 18, 2016. Reuters


“There is absolutely no way that one could not recognize SAP material,” a former senior law enforcement official with experience investigating violations of SAP procedures told Fox News. “It is the most sensitive of the sensitive.”

Controversy over Clinton’s use of a private email server during her tenure as secretary of state from 2009 to 2013 has plagued her all election season. In response to a Freedom of Information Act lawsuit, a judge ordered the State Department to release a batch of Clinton’s emails at the end of every month from June 2015 to January 2016. Many of the emails have been heavily redacted, and the last batch is expected to be released Jan. 29.