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.

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