WASHINGTON (AP) — The White House violated federal law in withholding security assistance to Ukraine, an action at the center of President Donald Trump’s impeachment, a federal watchdog agency said Thursday.

The Government Accountability Office said in a report that the Office of Management and Budget violated the law in holding up the aid, which Congress passed less than a year ago, saying “the President is not vested with the power to ignore or amend any such duly enacted law.” The aid in question was held up last summer on orders from Trump but was released in September after a whistleblower’s complaint became public.

The independent agency, which reports to Congress, said OMB violated the Impoundment Control Act in delaying the security assistance Congress authorized for Ukraine for “policy reasons,” rather than technical budgetary needs.

“Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law,” wrote the agency’s general counsel, Thomas Armstrong, in the report.

Capitol Hill Democrats seized on the report as evidence of a lawless White House. led by Acting Chief of Staff Mick Mulvaney, who is a key figure in the impeachment investigation of Trump. He is still officially the OMB director.

“Congress makes funding decisions, and the Trump Administration’s illegal impoundment of these vital national security funds was a brazen assault on the checks and balances inherent to our democracy,” said House Appropriations Committee Chairwoman Nita Lowey, D-N.Y. “Given that this illegal conduct threatened our security and undermined our elections, I feel even more strongly that the House has chosen the right course by impeaching President Trump. No one is above the law.”

OMB has argued the hold was appropriate and necessary.

“We disagree with GAO’s opinion. OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President’s priorities and with the law,” said OMB spokeswoman Rachel Semmel.

Trump was impeached last month on charges of abusing his power for pressuring Ukraine to investigate Democratic rivals, as he was withholding the aid, and for obstructing Congress’ ensuing probe. The Senate is set to begin its trial on Thursday.

The GAO finding concludes that the White House budget office “withheld the funds for an unauthorized reason in violation” of the Impoundment Control Act, a federal law that requires the executive branch to spend money that is appropriated by Congress.

The impoundment control law is rigorously adhered to by career officials in agency budget offices, who can face severe trouble for violating it.

Read the full GAO report.