In a letter to members of Congress on Thursday, the Inspector General of the Intelligence Community concluded that Mrs. Clinton’s email contains material from the intelligence community that should have been considered “secret” at the time it was sent, the second-highest level of classification. A copy of the letter to Congress was provided to The Wall Street Journal by a spokeswoman for the Inspector General.

The four emails in question “were classified when they were sent and are classified now,” said Andrea Williams, a spokeswoman for the inspector general. The inspector general reviewed just a small sample totaling about 40 emails in Mrs. Clinton’s inbox—meaning that many more in the trove of more than 30,000 may contain potentially secret or top-secret information.