WASHINGTON — The White House's decision to block the release of almost $400 million in aid to Ukraine for more than two months offers a potentially fertile paper trail that House Democrats believe could bolster their case for impeachment against President Donald Trump, congressional sources and former U.S. officials told NBC News.

House Democrats leading the impeachment inquiry issued subpoenas Monday to the Office of Management and Budget (OMB) and the Defense Department, hoping to get their hands on documents that could shed light on how White House officials sought to hold up the funding package, and how they communicated the decision to the Defense and State departments.

Actions taken to freeze or delay the release of $400 million likely would have been accompanied by a trail of written communications within the OMB and between the White House and the Pentagon and the State Department, according to former U.S. officials and congressional sources.

Under federal law, U.S. presidents have little flexibility when it comes to holding up spending funds approved by Congress. Civil servants handling federal funds at the OMB and the Defense Department are keenly aware of the law's provisions, which require the executive branch to explain why it is delaying the release of monies and that the rationale meets strict criteria.

"Given the administration was putting career officials between a rock and a hard place, I would be very surprised if they didn't take steps to document their concern," said Sam Berger, a former senior official at the OMB and now vice president at the left-leaning Center for American Progress think tank.

"This would be a huge red flag for any career official involved."

But it's not at all clear the Trump administration will comply with the subpoena request, one of a series issued in recent days, as White House officials — and most Republicans in Congress — have dismissed the impeachment inquiry as a politically motivated witch hunt.

The delay in the security funds is at the center of the impeachment inquiry, which is examining whether Trump pressured Ukraine to conduct investigations for his own political gain and allegedly held up the aid as leverage. The role of the OMB in handling the aid money has also placed Mick Mulvaney, the director of the OMB and also the president's chief of staff, under growing scrutiny.

A senior administration official defended how the White House handled the military assistance funding for Ukraine, describing it as consistent with how other aid money for foreign governments has been reviewed.

"The president has consistently made it clear that any foreign aid spent overseas must not only run through a good government process, but also protect U.S. interests abroad," a senior administration official told NBC News. "That good government process was run by the president's policy team on this account to ensure that those goals were met."

The official said the administration followed federal law and that the approach was in keeping with the president's focus on making sure foreign aid programs protect U.S. interests abroad and ensure allies contribute an equitable share.

In text messages between U.S. diplomats released last week by the House Democrats, the acting ambassador in Kyiv, William Taylor, expressed concern that the military aid was delayed to push Ukraine to launch a probe to help with Trump's re-election campaign.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor wrote to Gordon Sondland, the U.S. ambassador to the European Union.

Replying to Taylor, Sondland wrote that Taylor was "incorrect" about Trump's intentions and that there was no quid pro quo.

The aid for Ukraine included $250 million to be managed by the Defense Department, including Javelin anti-tank weapons for Ukraine's armed forces. In June, the Pentagon announced plans to provide the funds for Ukraine, but the money never materialized.

Lawmakers asked the Pentagon and the State Department why the funds were delayed and never got a clear explanation, congressional aides told NBC News.

According to a whistleblower complaint, OMB officials on July 18 informed government departments that Trump earlier that month had suspended security assistance funds to Ukraine but were not aware of the rationale for the decision.

Federal law requires the executive branch to inform Congress why certain funds aren't being released, according to Berger and other former officials who worked on budget issues.

Democratic Sen. Chris Coons of Delaware, a member of the Senate Foreign Relations Committee, said he and his fellow lawmakers were puzzled about the holdup, and were embarrassed when Ukraine's President Volodymyr Zelenskiy asked why the aid had not been delivered.

"Republicans and Democrats have both expressed frustration about why our security assistance to Ukraine was delayed, and when some of my colleagues on Foreign Relations met with President Zelenskiy in September, they didn't know how to respond when Zelenskiy asked why such urgently needed assistance was being withheld," Coons told NBC in an email.

Usually, OMB will delay or defer the release of funds because the price of a particular project is lower than originally estimated, or an agency has been asked to submit a more detailed spending plan, according to G. William Hoagland, vice president of the Bipartisan Policy Center think tank and a former senior Senate staff member who worked on budget policy.

"It's a straightforward administrative issue," Hoagland said, with civil servants at OMB handling the deferral.

Funds appropriated by Congress cannot be held up due to policy disputes, Hoagland and other former officials said.

Trump has offered different explanations as to why the funds were delayed, saying that he wanted European governments to provide more assistance to Ukraine and that he wanted Ukraine to take more action to fight corruption. While Trump and other officials have been outspoken in their criticism of NATO allies over the level of their defense spending, the administration had not publicly complained about a lack of burden sharing when it comes to aid for Ukraine.

As for Ukraine tackling corruption, Undersecretary of Defense for Policy John Rood informed Congress in May that he had "certified that the Government of Ukraine has taken substantial actions to make defense institutional reforms for the purposes of decreasing corruption (and) increasing accountability."

The hold on the aid was lifted in September, days before the end of the fiscal year on Sept. 30. Defense Secretary Mark Esper said last month that the delay did not cause any damage to U.S. national security.

"I'm pleased to say we were obligated to have that money out the door by the end of the fiscal year," Esper said. "At this point, most of the money is out the door. And at no time or at any time has any delay in this money, this funding, affected U.S. national security."

The former U.S. envoy to Ukraine, Kurt Volker, whose role in Trump's approach to Ukraine has become a focus of the impeachment inquiry, told lawmakers last week in his opening remarks that he became aware of the hold on the aid in July. But Volker said he was not "overly concerned" at the time, because Congress and officials across the federal government favored releasing the assistance package to Ukraine.

"Everything from the force of law to the unanimous position of the House, Senate, Pentagon, State Department, and NSC staff argued for going forward, and I knew it would just be a matter of time."