It sounds like the beginning of a good whodunit, or at least a “Law and Order” episode: A body is found. In the victim’s pants are two notes written in some sort of code. The F.B.I. is called in and concludes the man was murdered.

But the encrypted notes have the F.B.I. stumped — so stumped that this week, after years of trying to decipher them, it posted what amounted to a public request for help on its Web site. The two notes, a jumble of letters and numbers occasionally set off with parentheses, have also flummoxed members of the American Cryptogram Association, which the F.B.I. has consulted.

The body of Ricky McCormick was found in 1999, in a cornfield west of West Alton, Mo.

The F.B.I., drawn into the case because of the two mysterious notes, came to believe that Mr. McCormick, 41, was murdered and that the encrypted notes might lead to the killer.

“We are really good at what we do,” Dan Olson, chief of the F.B.I.’s Cryptanalysis and Racketeering Records Unit, said in a statement on the site. “But we could use some help with this one.” The notes, he said, are part of one of the unit’s “top unsolved cases.”