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 Mrs. 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 and other secret programs run by the Pentagon and the C.I.A. — 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 Mrs. Clinton sent or received the emails, nor how many contained the classified information.