WASHINGTON — Intelligence officials reviewing emails on former Secretary of State Hillary Clinton's private computer server have found information they consider to be of a higher level of classification than "top secret," according to a letter sent to lawmakers last week by the intelligence agencies' inspector general.

The letter, dated last Thursday, says that some of the information in Clinton's emails has been determined to be "top secret/SAP." That designation is usually given to information about "special access programs" — often intelligence-gathering programs run by the Pentagon — that are among the government's most closely guarded secrets.

The letter to Congress, from I. Charles McCullough III, the inspector general for the nation's intelligence agencies, provides no specifics about the classified material. It is not clear from the letter whether Clinton sent or received the emails, nor how many contained the classified information.

Last year, the intelligence agencies found after the fact that two of Clinton's emails contained information that was "top secret." The State Department disputed the determination, but it prompted an FBI investigation into whether such information had been mishandled in connection with Clinton's account. By law, classified information is not allowed outside government computer servers. The government has said that Clinton is not a subject of the investigation.

The letter was sent to the Republican chairmen of the Senate Intelligence and Foreign Relations Committees. It was not made public.

A spokesman for Clinton's presidential campaign did not respond to a message seeking comment. In the past, she has said that she did not receive any emails that were marked as containing classified information.

John Kirby, the State Department spokesman, said Tuesday that the department "is focused on and committed to releasing former Secretary Clinton's emails in a manner that protects sensitive information."