Emails from Hillary Clinton's home server contained information classified at levels higher than previously known, including a level meant to protect some of the most sensitive U.S. intelligence, according to a document obtained by NBC News.

In a letter to lawmakers, the intelligence community's internal watchdog says some of Clinton's emails contained information classified Top Secret/Special Access Program, a secrecy designation that includes some of the most closely held U.S. intelligence matters.

Two American intelligence officials tell NBC News these are not the same two emails from Clinton's server that have long been reported as containing information deemed Top Secret...

The new revelation underscores the extent to which the email classification issue could continue to dog Clinton, as State Department and intelligence officials review sensitive information within messages that were blacked out before being released to the public.

Clinton, who tops national primary polling as a Democratic presidential candidate, has repeatedly said that none of the information she sent or received while secretary of state was marked classified, and nothing has emerged to contradict that. But it's become clear that classified information bled into the emails, which were sent over unencrypted channels open to interception by foreign intelligence agencies...

The declarations cover "several dozen emails containing classified information determined by the IC element to be at the CONFIDENTIAL, SECRET and TOP SECRET/SAP information."

An intelligence official familiar with the matter told NBC News that the special access program in question was so sensitive that McCullough and some of his aides had to receive clearance to be read in on it before viewing the sworn declaration about the Clinton emails.