Hillary Clinton stands attends the West Caucus Dinner in Las Vegas on Jan. 6. (John Locher/AP)

The intelligence community believes that at least several dozen of the emails sent and received by Hillary Clinton while she was secretary of state contained classified material, including intelligence material classified at the highest levels, according to a new letter by the inspector general.

Some of the information was classified "top secret/SAP" or "special access program," an even more restricted category, according to the letter.

In a Jan. 14 letter to congressional oversight committees, I. Charles McCullough III, the inspector general of the intelligence community, wrote that he had received sworn declarations confirming the presence of the classified material from intelligence officials who have taken part in reviewing Clinton's emails as they have been released publicly by the State Department.

The State Department has released 82 percent of Clinton's correspondence and is under a court order to make the remainder public by the end of this month. Prior to publication, the State Department has redacted passages of emails that its reviewers believe contain material that is classified. The department has indicated that about 1,300 of the emails released contained classified material but indicated that the vast majority were "confidential," the lowest classification level. The State Department has indicated that only a handful were "secret," the next higher level of sensitivity after confidential.

The new letter would suggest that the intelligence community believes many more of the emails that Clinton routed through a private server installed at her suburban New York home included information that should have been properly classified at the highest levels of secrecy. However, McCullough did not break down how many emails he has been told fell into each level of classification.

[FBI looking into the security of Clinton email server]

The letter was first reported by Fox News. A Clinton campaign spokesman did not immediately respond to a request for comment.

The State Department and the intelligence community have clashed in recent months over the proper classification level of the Clinton emails. With Clinton repeatedly noting that none of the emails contain information that was marked classified at the time they were sent, State officials have typically indicated that the emails have been "upgraded" to a classified status as part of the review process to prepare them for public release.

However, the IC watchdog has been insistent since the summer that some of Clinton's correspondence contain information that was classified at the time it was sent and remains so today. It is the responsibility of individual government officials to properly mark classified material in their correspondence and to transmit it only through secure channels.

State Department spokesman John Kirby declined to comment on the specifics of the letter at a news briefing on Tuesday.

"What I'll tell you is that we are focused on — and remain focused on — releasing...the remainder of former secretary Clinton's emails in a manner that protects sensitive information," he said. "Our FOIA review process is still ongoing, and once that process is complete, if it is determined that information should be classified as top secret, then we'll do so."

Clinton's use of a private email account while secretary of state has become a less politically potent issue for her since Sen. Bernie Sanders, her main rival for the Democratic presidential nomination, declared at a debate in October that Americans were "sick and tired" of hearing about her "damn emails."

Still, the issue has continued to simmer, as an FBI investigation into the security of the private server proceeds and the intelligence community continues to review the content of Clinton's emails. Even after the last batch of Clinton's emails is released at the end of January, the issue will not entirely die: A federal judge has ordered the State Department to begin releasing in March batches of emails sent and received by top Clinton aide Huma Abedin to the conservative group Judicial Watch, which had sought them using a public information request. Abedin is the only other State Department official known to have used an email account managed through Clinton's private server.