We want the truth, the whole truth. The best way to get it is to immunize congressional witnesses who tell the story. That way, they will be part of a common effort to understand what went wrong, rather than targets of a criminal investigation. It may surprise you to learn that there are many civil servants who have quietly asked to be subpoenaed so their superiors will not accuse them of ratting out their bureaucratic cohorts. Fair enough. Subpoena them, and give them the protection of immunity, provided they tell the truth.
The cudgel of criminal action should be reserved for those who insist on clamming up, for they are obstructing investigations desperately needed to protect and advance our freedoms. But, as Andy McCarthy argues with his usual brilliance, for heaven's sake let's not have special prosecutors, who will throw a mantle of secrecy ("can't talk about it, there's a criminal investigation here") over the whole thing. We want transparency, not secrecy. Secrecy is part of the problem and no part of the solution.