A popular argument for why Scooter Libby should never have been prosecuted is that Patrick Fitzgerald knew early on in the investigation that Richard Armitage at the State Department was the leaker. If Fitzgerald knew Armitage was the leaker, why didn’t he stop the investigation right away? Why did he continue? For some people, Fitzgerald’s decision not to close up shop after learning Armitage was the leaker proves that he was an overzealous prosecutor run amok. He must have had some irrational desire to go after Libby, the argument runs, making the entire Libby prosecution unfair from the get-go.
I don’t find this argument persuasive. To see why, imagine yourself in Fitzgerald’s shoes. Here are the relevant facts as you know them (reconstructed as best I can — please let me know if these facts are misleading or wrong and I’ll correct them). You’ve been appointed a special prosecutor to investigate intentional leaks to the media of the covert identity of a CIA agent. Early on in the investigation, you learn that one high-level political official has admitted that he leaked Plame’s identity to one reporter; he claims that it was an accident, as he didn’t realize the agent’s status was covert. You also know that a lot of other reporters were leaked the same information, but you don’t know who was behind those other leaks. The reporters won’t talk: They insist on going to jail rather than revealing their sources.
If you were Fitzgerald, would you close up shop at that point? Would you conclude without even speaking to other potential witnesses that the one high-level official was in fact responsible for all the leaks, and that he acted accidentally and entirely on his own? Or would you at least want to dig deeper to see if the story checks out?
  In that setting, I don’t understand what was so overzealous about wanting to talk to Libby. An experienced prosecutor is going to wonder if the guy who rushes forward and claims the leaks were an accident is telling the truth. Maybe he is. But you don’t want to close up shop and then read in someone’s memoirs ten years from now that the official (Armitage) was the fall guy who came up with the “accident” story to cover up something — and that he got away with it because the naive prosecutor bought the story and closed the investigation without even verifying the facts. Or maybe someone was using Armitage as an unknowing intermediary, making his story accurate from his perspective but only part of the picture. Or maybe there were other leakers — either more leakers to the one reporter (Novak) who reported to the public about Plame, or other leakers to the other reporters. None of these are certainties, of course. But it is really so unreasonable to look into them?
To repeat myself from yesterday, I’m certainly open to argument that Libby’s punishment was too high, or that there were parts of the case that were unfair. But I don’t understand why Fitzgerald’s wanting to speak to Libby is proof that he was an overzealous prosecutor run amok. What am I missing?