The prosecutor is often first presented with a case as a “corpus delicti” — a bullet-riddled body in the street, for instance. That ordinarily is enough to justify investigation. Through investigation, the evidence may prove that there was not in fact a crime (it was a suicide or an accident) or that the fatal acts were privileged or enjoy a legal defense (self-defense or justifiable shooting by an officer of the law). But one begins by investigation.
The judicial branch (which, under Marbury v. Madison, has the ultimate duty to determine “what the law is”) has determined that waterboarding is torture (see U.S. v. Lee, decided in 1984 by the U.S. Court of Appeals for the 5th Circuit). The Bush administration has admitted to waterboarding captives. The corpus delicti of that crime exists. For there to be investigation now is unexceptional.
