(Peter Jennings) Finally here this evening, the embarrassing secret of the CIA. Over the years the Central Intelligence Agency has studied and broken some of the toughest codes ever created. Tonight it's top code breakers are stumped by a puzzle that is quite literally staring them in the face every day. Here's ABC's John Martin.

(John Martin)[CU LETTER] It is called cryptos, Greek for hidden. It is a curved copper screen of letters in secret code. [MONUMENT] It was dedicated five months ago at CIA headquarters. So far apparently the agency's code breakers can't figure out what it says.

(James Sanborn) Once the plate is deciphered I'm not convinced the true meaning will be clear even then. There's another deeper mystery.

(John Martin) Sculptor Sanborn created the mystery after he won a 250,000 dollar commission three years ago. He says he decided to create a work that touches the hidden nature of the CIA. [JIM SCULPTING] He refuses to say what the message is. Could it have perhaps a subversive quality?

(James Sanborn) It could corrupt somehow. It might cause...that people at the agency to perhaps think of things a little bit more, less seriously.

(John Martin) [STUDIO EXT/INT] Somebody, perhaps from the CIA, took the sculpture very seriously. [CIA BROCHURE]

(James Sanborn) A lot of strange things happened while I was doing the piece. There were people caught on ladders trying to look in my window and photograph the piece through my windows and they were run off by the police.

(John Martin)To code his secret, Sanborn turned to a retired CIA cryptographer. Today, the CIA refuses to discuss the code in public. It insisted that Sanborn hand CIA Director William Webster an envelope containing the code and the message. At its dedication Webster praised the sculpture and called the code a test. So far nobody has passed it.

(James Sanborn) Sure sombeody will figure it out eventually and then personalities will change, you know. Ten years will go by, fifteen years will go by and they'll forget what it says again.

(John Martin) [SU] So an agency often synonymous with secrecy in the world has collected one more secret message to go along with the billions already here, confident that it can decipher it and if necessary keep it from the outside world. John Martin, ABC News, Langley, Virginia.

(Peter Jennings) That is our report on World News Tonight. I'm Peter Jennings. Have a good evening, we'll see you tomorrow, good night.