The decision was made out of concern that such a probe could duplicate or interfere with the impeachment inquiry that took place in the House of Representatives, according to a letter sent to senators on Wednesday.
A spokesman for the Defense Department inspector confirmed that they had sent a letter to several top Democratic senators regarding the decision but noted that "as circumstances change our decision may change."
Democrats requesting review express their frustration
"We are disappointed that the Defense Department's Office of the Inspector General has refused to investigate the Defense Department's delay in delaying the Ukraine Security Assistance Initiative provided by Congress , "Sen said. Dick Durbin, an Illinois Democrat, said in a statement.
He and several other leading Democrats wrote to the general inspector general in September asking him to investigate the delay, citing concerns that "it would appear to hinder the law contained in Department law security assistance to Ukraine at a critical moment and raises serious questions about whether DoD officials are involved in any scheme to target a political opponent. "
" As a result, it is necessary that your office will pursue a thorough review of the DoD's potential role in these allegations, and present your findings to congressional defense committees in a timely manner, "Durbin wrote in a letter to the Inspector's acting General Glenn Fine.
The Democratic House conducting an impeachment investigation argued that the President had attempted to use the aid as a seizure while forcing Ukraine to investigate local political rivals, evidence, they say, of a quid pro quo .
The White House offered changing and vague accounts of why the hold was implemented and what prompted Trump to finally change course and release the money.
Administration officials have tried to pin down the delay in government bureaucracy. They point to reviews by national security agencies and another Office of Management and Budget to ensure funds are properly spent. But detailed reviews were conducted before the funds were frozen in June. And there is little indication that any such process has occurred after freezing.
He claimed that Volker had told him at their meeting that he was trying to increase the hold by having Ukrainians deliver a public statement that they would launch investigations that Trump was seeking.
The Office of Government Accountability began the review after Sen. Chris Van Hollen, a Maryland Democrat, asked the US comptroller if the administration violated the law by holding funds until "the last minute" and failing to inform Congress of the decision, according to office spokesman Charles Young.
CNN & Ryan Browne contributed to this report.