It’s been a matter of extreme controversy since last December, when EPA — confronted with an impending front-page Washington Post exclusive — suddenly announced it was denying California’s request to enforce its greenhouse gas standards for motor vehicles.

After months of dogged investigation, California Rep. Henry Waxman disclosed today that he had evidence that the White House tampered with the decision. The issue is certain to come up tomorrow as EPA Admistrator Steve Johnson appears before Waxman’s panel.

Waxman’s House Committee on Oversight and Government Reform released the results of its investigation (PDF) today, including private depositions with key EPA staffers.

Waxman noted not only were key EPA staffers united in recommending that EPA Administrator Steve Johnson grant the waiver, but that Johnson himself appeared to be ready to grant at least a partial waiver (an idea devised by EPA staff in hopes of making it more palatable to the White House) — until the EPA chief communicated to the White House about it. After that, Johnson flip-flopped and announced he had rejected the waiver.

Key testimony came from EPA Associate Deputy Administrator Jason Burnett (PDF), who recently announced his departure from the agency. Burnett was deposed on May 15:

According to Mr. Burnett’s deposition testimony, Administrator Johnson’s preference for a full or partial grant of the waiver did not change until after he communicated with the White House. When asked by Committee staff “whether the Administrator communicated with the White House in between his preference to do a partial grant and the ultimate decision” to deny the waiver, Mr. Burnett responded: “I believe the answer is yes.” When asked “after his communications with the White House, did he still support granting the waiver in part,” Mr. Burnett answered: “He ultimately decided to deny the waiver.”

Waxman’s memo concluded that, "It appears that the White House played a significant role in the reversal of the EPA position." It added that, "It would appear to be inconsistent with the President’s constitutional obligation to faithfully execute the laws of the United States if the President or his advisors pressured Administrator Johnson to ignore the record before the agency for political or other inappropriate reasons."