Newly released emails between the Office of Management and Budget and the Pentagon regarding Ukraine defense aid held by the White House further reveal that a request to withhold funds came less than two hours after President Donald Trump's July phone call with the Ukrainian president that has served as the backbone of the impeachment proceedings against him.

"Based on guidance I have received and in light of the Administration's plan to review assistance to Ukraine, including the Ukraine Security Assistance Initiative, please hold off on any additional [Department of Defense] obligations of these funds, pending direction from that process," Mike Duffey, a political appointee serving as associate director for national security programs at OMB, wrote on July 25 to OMB and Pentagon officials.

The Center for Public Integrity obtained 146 pages of heavily redacted emails through a Freedom of Information Act request and court order. The nonprofit released the emails late on Friday, revealing a discussion between the White House's Office of Management and Budget and the Pentagon over the defense aid owed to Ukraine just hours after Trump spoke to President Volodymyr Zelenskiy.

Government officials raised concern over the much-discussed phone conversation as it appeared that President Trump improperly asked Zelenskiy to investigate Hunter Biden, the son of former Vice President Joe Biden — one of Trump's chief political rivals in the 2020 election.

The administration put a hold on critical defense aid for Ukraine as early as the week of July 18, one week before the phone call between Trump and Zelenskiy, at the direction of acting White House Chief of Staff Mick Mulvaney, two administration officials and a senior Democratic aide briefed by the State Dept. told NBC News in September. The funds were released on Sept. 11.

It appeared that those involved felt the hold could be problematic, considering Duffey wrote to officials in his office and at the Pentagon to keep it all close their chest.

"Given the sensitive nature of the request," Duffey wrote on the afternoon of Trump's call with Zelenskiy, "I appreciate your keeping that information closely held to those who need to know to execute direction."

The emails also provide insight into administration officials' feelings once the funds are finally released, though their reasoning for distributing the aid suddenly appears to be redacted.

Elaine McCuskker, the Pentagon's comptroller, said there was "increasing risk of execution" in continuing to hold the funds.

Three House committees announced that they would launch a wide-ranging investigation into the allegations against Trump two days prior to the release.

On Sept. 11, Duffey shared his feelings of relief to McCusker a few hours after alerting her that he would be releasing all of the Ukraine Security Assistance Initiative funds.

"Glad to have this behind us," he said.

Duffey is one of the officials at OMB who has steadfastly refused to comply with House subpoenas for deposition in the impeachment inquiry, along with his boss, Russ Vought, OMB’s Acting Director. However, another OMB official, Mark Sandy, did appear for a deposition after being subpoenaed.

The Pentagon and OMB did not immediately respond to requests for comment by NBC News.

Nevertheless, the House voted to impeach Trump on Wednesday. He is only the third president in U.S. history to be impeached.

A trial in the Republican-controlled Senate to decide whether Trump will remain in office is expected to begin January, but Majority Leader Mitch McConnell, R-Ky., said Thursday that he has reached an “impasse” with Democratic leaders over moving forward with the rules governing the trial.

It is likely Trump will be acquitted, as it would require a two-thirds majority for a conviction.