I GET THE LOGIC BUT....

AG Mukasey said the DOJ would not investigate waterboarding or warrant-less wiretapping because the DOJ's Office of Legal Counsel already judged that those practices were legal, according to the WaPo:

Waterboarding, he told the House committee, "cannot possibly be the subject of . . . a Justice Department investigation" because its use was approved by the Justice Department's Office of Legal Counsel. Mukasey made a parallel argument about the National Security Agency's warrantless wiretapping program, saying the Justice Department could not investigate that program because it was approved at the outset by the department's lawyers.

"That would mean that the same department that authorized the program would now consider prosecuting somebody who followed that advice," he said.

The problems here are that OLC has been compromised by political hacks and, more broadly, the Vice President has subverted the normal review process. You can find more discussion about Mukasey's testimony here.