This is why Waxman subpoenaed James Comey.

Behind the Scenes of the U.S. Attorney Firings

But in the interest of the “betterment of the organization,” Sampson agreed to begin an informal evaluation of U.S. attorneys. Indeed, over the next year, Sampson began to “consult” with senior officials in an attempt to identify the 10 worst U.S. attorneys who could conceivably be replaced. Current and former Justice officials say Sampson consulted with, among others, Michael Battle, head of the Executive Office of U.S. Attorneys–who has now resigned–and his predecessor, Mary Beth Buchanan, as well as then Deputy Attorney General James Comey, who’s now senior vice president and general counsel of Lockheed Martin. Comey would confirm to U.S. News only that he provided Sampson with a list of weak U.S. attorneys.

But a former Justice official says that Comey’s list bore little resemblance to the list of those fired last year.

The only prosecutor on the fired list who also was on Comey’s list was Kevin Ryan, in San Francisco, who, the Washington Post reported Tuesday, had “widespread management and morale problems in his office.” In fact, Ryan wound up on Sampson’s list among the higher-ranked prosecutors.

Finally, a year after his initial conversation with Miers, Sampson wrote her an E-mail informing her that there were "practical obstacles to removing and replacing U.S. attorneys." Among them:

"Significant disruption" to the work of the Justice Department;

Raising the ire of home-state senators, who have customarily been consulted on U.S. attorney appointments;

Finding good replacements and getting them through Senate confirmation.

Update: Comey is set to testify in an open hearing this Thursday at 9:30 AM.