The inspector general's office for the Department of Defense is declining to open an investigation into the department's delay in providing military assistance funds to Ukraine, but will leave the door open to beginning one in the future.

The department's lead inspector announced the decision in a letter sent to Sen. Dick Durbin, D-Ill., on Tuesday night, on the eve of the first day of public impeachment hearings.

Durbin and a group of senators requested that the inspector general open an investigation into the delay in September, then again earlier this month. In his two-page letter, Acting Inspector General Glenn Fine noted that the same topic is one of the key issues in the impeachment inquiry underway in the House of Representatives.

"We seek not to duplicate or interfere with the efforts of other oversight entities, even if there is not a criminal proceeding, before initiating our own investigation," Fine wrote.

Fine served as the inspector general for the Department of Justice for 11 years, when he ran some investigations that overlapped with congressional reviews. But, his letter said, those investigations were typically initiated before the congressional action.

He also noted that the impeachment inquiry "is not a typical congressional oversight investigation."

For those reasons, the inspector general chose not begin the probe, but noted that that could change as the inquiry moves forward. After it concludes, the letter closed, "we would again consider investigating such DoD matters that have not been sufficiently addressed."