If the prosecutor knew or must have known going in that no crime had been committed (and there would be no crime if a necessary element was wholly incapable of being satisfied) then any indictment or prosecution, even a derivative indictment or prosecution for an offense such as obstruction of justice or perjury, would be subject to dismissal for abuse of process and prosecutorial misconduct. This would also give rise to a civil claim for wrongful or malicious prosecution and perhaps a constitutional claim against the prosecutor personally for denying the accused of due process under color of law.
Do you see any other crime that "caused" the investigation, other than the illegal revelation of the identity of a person whose identity was protected by law?
Not to. beat. dead. horse. But if Plame's id was not protected (because she was not covert within the meaning of the law), seems like that's a necessary element that could never be met.