The Justice Department’s own investigators issued a stinging rebuke of former Attorney General Alberto Gonzales and his top lieutenants Monday, concluding that they used political calculations to determine which U.S. attorneys were fired in December 2006 and then provided misleading explanations about the firings to Congress and the public.

The report, issued jointly by the department’s inspector general and the Office of Professional Responsibility, suggested that it could not fully determine how decisions were made to fire certain U.S. attorneys because the White House refused to provide some documents to the investigators and because some witnesses refused to cooperate. The investigators suggested that a special counsel be established to complete the investigation and to “determine whether the evidence demonstrates that any criminal offense was committed.”

Despite failing to draw conclusions on legal culpability, the report heaped criticism on Gonzales and his top aides. Gonzales had said that he delegated the evaluation of U.S. attorneys and the decisions about whom to fire to Kyle Sampson, his chief of staff, and that he had little involvement in the process.

But the report concluded that “this was not a simple personnel matter that should be delegated to subordinate officials — it was an unprecedented removal of a group of high-level Department officials that was certain to raise concerns if not handled properly.”

Gonzales did not tell Sampson how to evaluate the U.S. attorneys, did not offer criteria by which they should be judged and said that even after the U.S. attorneys had been fired, he was unsure why some of them had been removed.

“Most remarkably,” the investigators wrote, “Gonzales told us he had ‘no recollection’ of the November 27, 2006, meeting in his conference room during which he approved the plan to request the resignations of the U.S. Attorneys,” despite having been given a three-page memo on the matter and giving his personal approval for the firings.

The report also states that Gonzales “made a series of statements after the removals that we concluded were inaccurate and misleading.” Among other things, Gonzales “repeatedly testified that the removals were not undertaken for an improper or illegal purpose. However, he could not have known whether that was true because he did not ask Sampson why the U.S. Attorneys were being removed.”

House Judiciary Chairman John Conyers (D-Mich.) has scheduled a hearing on the report for Friday.