From TV Guide Magazine:
"West Wing fans, brace yourselves: The show's White House national security leak is not who you think. And you'll never guess why. "When they subpoena vice presidential candidate Leo McGarry, the pressure becomes so intense that the character who leaked the information comes forward," says producer Alex Graves. As a result, adds exec producer John Wells, "a major character will leave the White House and the campaign trail." Chief of Staff CJ Cregg has been tagged as the main suspect . TV Guide knows who the culprit is. But, unlike Karl Rove, we'll never tell."