(CNN) A political appointee at the Office of Management and Budget took the unusual step of getting involved in signing off on freezing US aid to Ukraine this past summer -- a process normally reserved for career budget officials, according to sources familiar with the matter.

Michael Duffey, OMB's associate director for national security programs and a Trump political appointee, signed at least some of the documents delaying aid to Ukraine, two sources told CNN. Normally a career budget official signs such documents. Sources told CNN it is highly unusual for a political appointee to be involved in signing off on such a freeze.

In this case, career budget officials raised concerns about signing the documents because they believed such a move may have run afoul of laws requiring OMB to spend money as it is appropriated by Congress, according to a congressional aide.

Duffey's role is of interest to House Democrats who are conducting an impeachment inquiry over Trump's moves to pressure Ukraine for help investigating former Vice President Joe Biden and his son, Hunter Biden. There is no evidence of wrongdoing by either.

Congressional impeachment investigators believe that there may be a paper trail at OMB that sheds light on the decision to block aid to Ukraine this summer as Trump and his allies were pressuring the new government. The decisionmaking behind the administration's moves on aid has been obscured from both Republican and Democratic lawmakers.

The Wall Street Journal first reported that Duffey's involvement is of interest to the impeachment inquiry.

"The idea that administration officials would not be involved in budget execution, including apportionment authority, after decades of precedent, is absolutely ludicrous," said Rachel Semmel, a spokeswoman for OMB. "It is absurd to suggest that the President and his administration officials should not play a leadership role in ensuring taxpayer dollars are well spent."

Another source familiar with the situation said there was a legitimate reason for Duffey to personally sign off on the freeze. Relatively new to OMB, Duffey wanted a better understanding of how the apportionment process worked, a source said. The source said Duffey signed the paperwork to halt the aid based on his belief that the White House would want to review it because the President doesn't like spending on foreign aid in general.

"This is a highly unusual set of circumstances that would have raised serious red flags for career officials at the Department of Defense, the State Department and OMB," said Sam Berger, a vice president at the left-leaning Center for American Progress and a former senior counselor and policy adviser at OMB.

Congressional investigators looking to follow the money -- or rather, where it was frozen -- have so far hit a wall at OMB.

OMB's acting director Russell Vought made it clear Wednesday that he's prepared to block requests for information from House Democrats, in line with the White House position.

"We will not be participating in a sham process that's designed to relitigate the last election," Vought said on Fox News.

The documents that Duffey signed -- known as apportionments -- are part of the normal protocol at OMB and would reveal when military aid to Ukraine -- intended in part to help the country counter Russian aggression -- was halted and what explanation was offered.

The apportionment process isn't designed as a tool to carry out policy priorities or to advance political interests. If directed to freeze aid to Ukraine, OMB staffers may have been concerned they were running afoul of the law by not spending the money as appropriated.

There were other signs this OMB process on Ukraine didn't play out in the usual fashion.

For one, the freeze had been ordered directly by Trump in June, according to an administration official. After receiving the directive, OMB sat on the funds for months. And it provided few, if any, updates to lawmakers, the State Department and the Pentagon, causing widespread confusion about why the money had not been spent.

At a staff briefing in mid-September, State Department officials confirmed that they no objection to the Ukraine funding going forward. They pointed to Mick Mulvaney, Trump's chief of staff and the OMB director, as the person who directed the State Department not to obligate the funds.

The confusion across departments raises the likelihood that a trail of communication exists showing officials at the State Department and the Defense Department engaging OMB officials with questions or concerns about the hold up in funds.

"We should assume there are contemporaneous documents to reflect agency viewpoints," Berger said.

The circumstances surrounding the Ukraine funding were also bizarre because the Trump administration provided such a wide variety of reasons to explain the delay.

Officials have cited Trump's ideological objections to foreign aid, which were irrelevant once Congress appropriated the funds and then refused the Trump administration's follow up attempt to try to cut billions in foreign aid to a number of countries.

The administration has pointed to Trump's desire to have US allies to contribute more toward Ukraine, but the funds remained frozen even after Trump returned from the G-7 in August, a prime opportunity for diplomatic arm-twisting.

Another administration talking point has been that Trump wanted to crack down on corruption in Ukraine. But both the State Department and the Pentagon had signed off on sending the money to Ukraine after conducting an assessment that included a corruption check.

Finally, the administration has pointed to a mysterious interagency "policy review" as one of the hold ups. However, no officials will explain what was actually under review or why such a review was necessary after both the State Department and the Pentagon had signed off on the funds.

The one explanation the Trump administration rejects: That Trump may have halted money to Ukraine to pressure Ukrainian officials to investigate the Biden family. The President has vehemently denied there was any quid pro quo offered to Ukraine.

To the extent a paper trail does exist, lawmakers will have to put up a fight to get their hands on it.

OMB has provided some documents in response to recent requests from the House Budget Committee and House Appropriations committee, most likely because those were being requested for legislative and oversight purposes and not as part of the impeachment inquiry, a congressional aide said.

OMB faces another deadline Friday to hand over another set of documents to those committees, but sources told CNN they wouldn't be surprised if the budget office now refuses to hand over any documents to any committee for any purpose.

Asked repeatedly Wednesday whether a paper trail exists at OMB, Vought dodged the questions and offered up a non-response instead.

"OMB continues to do the job that is statutorily required to manage the people's money in a way that's consistent with the law and on behalf of the priorities of the President," Vought told Fox, "there's no question about that."