"We did not substantiate any of the allegations," the report said. "We determined Mr. Shanahan fully complied with his ethics agreements and his ethical obligations regarding Boeing and its competitors.''
The OIG explained that in mid-March it initiated an investigation into allegations that Shanahan took actions to promote Boeing and disparage its competitors in violation of his ethics obligations.
As part of the probe, the OIG interviewed Shanahan and 33 other witnesses, including the most senior Defense Department officials, individuals who had frequent interaction with Shanahan and others who were involved in the review, consideration, or decisions to purchase or budget for Boeing.
"We reviewed more than 5,600 pages of unclassified documents and approximately 1,700 pages of classified documents related to the allegations and the relevant major defence acquisition systems," the report said, adding that all interviews were conducted under oath and transcribed.
Shanahan was appointed Deputy Defence Secretary in the summer of 2017 and is widely expected to succeed Jim Mattis as the full Defence Secretary. However, the probe reportedly caused the White House to delay his nomination until the conclusion of the investigation.