The US Government Accountability Office (GAO), an agency tasked with auditing the federal government and the legality of its activities, found Thursday that the freeze on military aid to Ukraine that has been at the center of impeachment proceedings into President Donald Trump was unlawful. The agency’s analysis found that when the Office of Management and Budget withheld funds that had been apportioned by Congress, it violated the 1974 Impoundment Control Act (ICA).

“Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law. OMB withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA). The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA,” the decision states. “The Constitution grants the President no unilateral authority to withhold funds from obligation.”

The decision comes on the same day that the Senate begins trial proceedings in the impeachment of Trump. Charges of Trump’s abuse of power include accusations that he ordered the freeze on military aid to Ukraine, and withheld a meeting with the Ukrainian president, while seeking to get the Ukrainians to take public steps that would benefit his reelection. The Trump administration has asserted in recent months that nothing about the way they delayed the funds was untoward, and Thursday’s report may create new challenges for Trump’s impeachment defenders.

A spokesperson for the Office of Management and Budget responded to the GAO report and disputed its findings.