The new $250,000 sculpture decorating the inner courtyard at CIA headquarters contains a secret, coded message so difficult to unscramble that it's spooking the spy agency's smartest spooks.

Sculptor Jim Sanborn wrote the message but steadfastly refuses to divulge its contents. He says the text is known only to himself and CIA Director William H. Webster, and neither is talking.Sanborn's encrypted brain-teaser is etched in 2,000 letters on the curving copper plates of his outdoor sculpture, "Kryptos." The work was erected outside the mirrored windows of the CIA employee cafeteria, offering a tantalizing challenge to the lunchtime crowd inside.

"Everyone wants to know what it says," Sanborn says. "They're out there all the time. There are groups of dark-suited people pointing at it and getting down on their knees trying to figure out what it says. Some take photographs. One guy copied the whole thing down with pencil and paper."

Months ago, as rumors of the mystery sculpture swept through the corridors of the CIA's wooded enclave in suburban Langley, Va., police caught two men climbing ladders outside Sanborn's Washington studio trying to photograph the inscription through his windows.

Sanborn says "a friend of a friend" told him recently that frustrated CIA operatives sent a copy of the coded message to their arch rivals at the National Security Agency to run through their ultra-smart Cray supercomputer.

"The last I heard, they hadn't gotten very far," Sanborn said. "They might have figured out one-fourth of it."

The sculptor, a big, bearded 45-year-old who wears aviator's sunglasses, talks freely about his other works, which have been exhibited at the Smithsonian's Hirshhorn Museum and the Corcoran Gallery of Art in Washington.

But when he discusses "Kryptos" - which is Greek for "hidden" - Sanborn retreats behind the cautious, elliptical speech of somebody who's trying to hide something.

He said the sculpture's message deals on several levels with the CIA's tradition of secrecy. He hints that it was a "collaborative project," possibly written with the help of a prominent spy novelist, but he wouldn't elaborate.

The inscription includes a Vigenere table, a system of ciphers devised by the 16th century French cryptographer Blaise de Vigenere, and a Vigenere-coded message that Sanborn says an expert can decipher in a few hours.

But the rest of the message, contained in the lower-right quadrant, is "a whole different ball game" of multiple codes, Sanborn said. It was written by a retired CIA cryptographer whom the sculptor refuses to identify.