Washington(CNN) The US Defense Department Inspector General's Office is declining to open an investigation into why military assistance to Ukraine was delayed by the Trump administration.

The decision was made out of concern that such a probe could duplicate or interfere with the impeachment inquiry taking place in the House of Representatives, according to a letter sent to senators Wednesday.

A spokesperson for the Defense Department inspector general confirmed that they had sent a letter to several top Democratic senators regarding the decision but noted that "as circumstances evolve our decision could change."

Democrats who had requested the review expressed their disappointment.

"We are disappointed that the Defense Department Office of the Inspector General has declined to investigate the Defense Department's delay of the Ukraine Security Assistance Initiative funding appropriated by Congress," Sen. Dick Durbin, an Illinois Democrat, said in a statement.

He and several other top Democrats had written to the acting inspector general in September asking him to investigate the delay, citing concerns that it "would appear to have hindered the Department's statutory obligation to provide security assistance to Ukraine at a critical moment, and raises serious questions about whether DoD officials were involved in any scheme to target a political opponent."

"As a result, it is imperative that your office pursue a thorough review of DoD's potential role in these allegations, and provide your findings to the congressional defense committees in a timely manner," Durbin wrote in the letter to acting Inspector General Glenn Fine.

News that the Trump administration had placed a hold on aid to Ukraine surfaced in late August, and the circumstances surrounding that decision have come under scrutiny as part of the House Democratic impeachment inquiry into President Donald Trump.

House Democrats conducting the impeachment probe have argued the President attempted to use the aid as leverage while pressing Ukraine to investigate domestic political rivals, evidence, they say, of a quid pro quo.

The White House has offered shifting and vague accounts of why the hold was implemented and what triggered Trump to ultimately change course and release the money.

What is clear, however, is that in the days leading up to Trump's decision to unfreeze the aid, there was a confluence of significant events. National security adviser John Bolton was fired the day before the President released the aid, and White House officials had recently been made aware of a whistleblower complaint that had been filed concerning Trump's behavior toward Ukraine.

Administration officials have tried to pin the delay on government bureaucracy. They have pointed to reviews by national security agencies and another by the Office of Management and Budget to ensure the funds were well-spent. But detailed assessments had been conducted before the funds were frozen in June. And there's little indication that any such process took place after the freeze.

Laura Cooper, the Pentagon's deputy assistant secretary for Russia, Ukraine and Eurasia, told lawmakers behind closed doors last month that she had met with then-US Special Envoy to Ukraine Kurt Volker in August to discuss the hold on the aid.

She said Volker had told her in their meeting that he was attempting to lift the hold by having the Ukrainians deliver a public statement that they would launch the investigations sought by Trump.

A congressional watchdog group announced earlier this month that it is conducting a review to determine whether the Trump administration violated appropriations law by halting the funds.

The Government Accountability Office initiated the review after Sen. Chris Van Hollen, a Maryland Democrat, asked the US comptroller whether the administration had violated the law by holding the funds until "the very last moment" and failing to notify Congress of the decision, according to the office's spokesman Charles Young.