The Pentagon’s Office of Inspector General (OIG) on Wednesday, Nov. 13, declined to open an investigation into the decision made by the White House to hold up military aid to Ukraine because it would overlap with the House’s impeachment inquiry.

In a letter to Democratic senators, the OIG wrote that the issues raised in the meetings and letters are “significant,” but a separate investigation would duplicate, interfere with, or otherwise impact the ongoing inquiry of the House.

“It is clear that there would be overlap in key witnesses and similar documents to review in any potential DOD OIG investigation and the House impeachment proceedings,” the OIG wrote.

“Inspectors General are required to, and regularly do, consider the impact of our work on other proceedings and ongoing oversight by other entities, including criminal matters or congressional reviews,” it added.

The OIG said the department would consider opening an investigation in the future “as circumstances evolve and the congressional inquiry proceeds.”

A group of senators led by Senate Appropriations Defense Subcommittee Vice Chairman Dick Durbin (D-Ill.), whose office released the OIG’s letter, said they are “disappointed” by the Pentagon’s decision not to open an investigation.

The three House committees—Intelligence, Oversight and Foreign Affairs—are currently leading the investigation into whether President Donald Trump pressured the Ukrainian President Volodymyr Zelenskiy to launch an investigation into former Vice President Joe Biden and his son Hunter in exchange for military aid. Both presidents said their phone call was proper and there was no “pressure” or “blackmail.”

During the first public impeachment hearing on Wednesday, key witness William Taylor, the acting U.S. Ambassador to Ukraine testified that the country was not aware of a hold on military aid until the end of August, more than a month after the conversation between two presidents Trump and Zelenskiy.