Washington (CNN) The White House budget office "impeded" part of the investigation into a controversial pro-industry decision that former Environmental Protection Agency Administrator Scott Pruitt pushed through on his way out the door, according to an inspector general's report released Thursday.

The EPA inspector general report concluded that EPA, and possibly the budget office, bypassed key steps when it rushed through Pruitt's proposal to lighten regulations on some heavy-duty trucks using older, less-efficient engines.

Pruitt resigned under a cloud of ethical questions, including allegations he wasted taxpayer funding on extravagant travel arrangements and an unprecedented security detail that surrounded him 24/7, and that he lived in a lobbyist's apartment at a cut-rate lease.

But the inspector general's office said it couldn't get all the information it wanted from the White House Office of Management and Budget.

The office "refused to provide ... specific responses or documentation," the inspector general said Thursday. The office told the inspector general that the information was "particularly sensitive."

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