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(NEWSER) – Jim Sanborn punched four messages into his curved copper sculpture, which has sat outside CIA headquarters in Langley, Va., since 1990. The first three messages of "Kryptos" were decoded by NSA cryptographers in 1993, then by fans in 1999. The 97-character fourth message, however, has stumped code breakers for 24 years, reports the New York Times.

Sanborn, 69, threw fans a bone in 2010 when he revealed the 64th to 69th characters spell out "Berlin," but no one cracked the puzzle. Now, he's offering another clue: The next part of the sequence—characters 70 through 74, which read "MZFPK"—spells out "clock."

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