Last month, in an interview with NBC News after revelations about the program, Clapper said: “I responded in what I thought was the most truthful, or least untruthful, manner” — because the program was classified.

In his letter to Feinstein, Clapper wrote that he was thinking about whether the NSA gathered the content of emails, rather than the metadata of the phone records — the record of calls to and from U.S. citizens and the length of those phone calls.

“I realized later Sen. Wyden was asking about … metadata collection, rather than content collection,” Clapper wrote. “Thus, my response was clearly erroneous, for which I apologize.”