What arrived was a three page letter. They started by acknowledging the request (how and when they received it) and assigned it a case number. They go on to state their mission is to, "collect, process, and disseminate communications or signals intelligence information for intelligence and counter intelligence purposes and to support military operations" and cite the executive order that defines their roles and responsibilities. The letter then addresses the, "considerable speculation about two NSA intelligence programs in the press/media."
I won't recite the next page and a half of legalese that follows however, it can be boiled down to one sentence:
"your request is denied because the fact of the existence or non-existence of responsive records is a currently and properly classified matter"
That's it in a nutshell. They are withholding the information not because the information may contain some classified secret but because I'm simply not allowed to know if they have it or not.
A copy of the letter is below: