President Donald Trump and his acting chief of staff, Mick Mulvaney. Jonathan Ernst/Reuters

A White House review of President Donald Trump's hold on military aid to Ukraine reportedly found hundreds of documents detailing efforts among senior officials to come up with a reason for the hold after it was already issued.

The Washington Post reported that emails between the acting White House chief of staff Mick Mulvaney and White House budget officials sought to establish a legal backing for Trump withholding the funds.

The White House Counsel's Office is concerned that documents surfaced in its research include "unflattering" conversations and facts that could present some political challenges for the president, according to The Post.

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A White House review of President Donald Trump's hold on nearly $400 million in military aid to Ukraine reportedly found hundreds of documents detailing efforts among senior officials to come up with a reason for the hold after it was already issued.

Citing three people familiar with the records, The Washington Post reported that the confidential investigation was sparked by the impeachment inquiry into Trump and detailed emails between the acting White House chief of staff Mick Mulvaney and White House budget officials searching for a legal backing for Trump withholding the funds.

The White House Counsel's Office is concerned that documents surfaced in its research include "unflattering" conversations and facts that could present some political challenges for the president, according to The Post.

The Post said that Mulvaney asked the acting Office of Management and Budget director, Russell Vought, about a possible legal explanation one month after Trump withheld the aid and that budget officials differed with National Security Council and State Department staffers over whether the move was legal at all.

The withheld aid is at the center of House Democrats' impeachment inquiry, which was launched after an anonymous whistleblower expressed concern over a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky to pursue two investigations that would benefit Trump politically.

Trump has sought to avoid linking his requests for those investigations to his decision to withhold the military aid.

Multiple US officials who worked with Ukraine testified to House investigators over the past few weeks that they disagreed with the decision to withhold the aid and were not given clear reasons it had been withheld.

Mark Sandy, a longtime Office of Management and Budget official, testified that the White House's decision to freeze the aid was "highly irregular."

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