Washington(CNN) The White House refused to provide documents to a nonpartisan congressional watchdog investigating President Donald Trump's decision to withhold US security aid to Ukraine, according to documents released by Democratic Sen. Chris Van Hollen of Maryland on Thursday.
Included in the release is a December 20 letter from the White House responding to an inquiry from the Government Accountability Office by citing a previous legal memo from the Office of Management and Budget defending the military aid freeze.
In the letter, Brian Miller, a senior associate counsel to the President, was responding to a request from the GAO seeking "factual information and legal views" about why the Ukraine aid had been held from acting White House chief of staff Mick Mulvaney and White House counsel Pat Cipollone. Miller informed the GAO that Mulvaney and Cipollone would not be participating in the probe.
"The White House does not plan to respond separately to your letters," he wrote.
News of the White House's refusal to participate in the GAO's investigation comes after the watchdog agency issued its decision earlier this month that the Trump administration had broken the law when it withheld the security aid to Ukraine last year, which had been appropriated by Congress.
The GAO concluded that the White House budget office had violated the Impoundment Control Act, a 1974 law that limits the White House from withholding funds that Congress has appropriated.
The Office of Management and Budget, which did provide a substantial response to the inquiry, had told the GAO it "withheld the funds to ensure that they were not spent 'in a manner that could conflict with the President's foreign policy,'" said Thomas Armstrong, the GAO's general counsel.
The GAO rejected that argument.
"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the GAO wrote. "OMB withheld funds for a policy reason, which is not permitted under the Impoundment Control Act. The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA."
After the report was released, OMB spokeswoman Rachel Semmel said the budget office disagrees with the GAO's decision.
"OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President's priorities and with the law," Semmel said.
Also included in Van Hollen's document release is a December 10 letter from the Pentagon citing an "interagency equities at issue" as the reason they couldn't provide the GAO a "substantive reply" to their inquiry.
"We are working diligently with our interagency colleagues to determine what, if any, information or answers we can provide," wrote Paul Ney, the General Counsel of the Department of Defense.
The GAO's ultimate conclusion undercuts much of Trump's defense in the ongoing Senate impeachment trial that no crime was committed in his conduct regarding Ukraine.
In releasing the documents on Thursday, Van Hollen tweeted that the White House's lack of cooperation was "part of a cover-up."
"As seen in these documents, OMB provided incomplete responses and the White House flat out refused," he said.
This story has been updated with additional information from the documents.