The Pentagon's Office of Inspector General (OIG) declined on Wednesday to launch an investigation into the decision made by the White House to hold up military aid to Ukraine, citing a desire not to interfere with the House's impeachment inquiry.

In a letter to Democratic senators, the OIG wrote that the Pentagon's investigative body would not open an investigation that duplicated or otherwise could interfere with an existing congressional inquiry.

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"It is clear that there would be overlap in key witnesses and similar documents to review in any DoD OIG investigation and the House impeachment proceedings," the Pentagon's OIG wrote.

"For these and several other reasons we discussed, we do not believe we should begin an investigation at this time," the OIG concluded, adding that an investigation could be opened in the future were the congressional inquiry to "evolve."

Sen. Dick Durbin Richard (Dick) Joseph DurbinThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Feinstein 'surprised and taken aback' by suggestion she's not up for Supreme Court fight Grand jury charges no officers in Breonna Taylor death MORE (D-Ill.), whose office released the OIG's letter, called the Pentagon's decision disappointing in a joint statement with several Democratic senators.

“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," the senators said.

"The withholding of this security assistance and the Department’s subsequent handling of information and materials related to the delay raises numerous questions and concerns. Inspectors General play a critical role in uncovering potential wrongdoing, ensuring accountability, and maintaining trust in our institutions of government," they continued. "We continue to believe that an investigation by the Defense Department Office of the Inspector General is important to get to the truth and ensure appropriate accountability.”

Democrats have argued that President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's decision to hold up military aid to Ukraine while pressuring the country's new president to launch an investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE (D) was improper, and have made the issue central to an impeachment inquiry formalized by the House last month.

Republicans have argued that aid to Ukraine was not held up on condition of an investigation being launched, while the president has defended his conversations with Ukraine's leadership.