The specific allegations that the Defense Department’s inspector general is examining revolve around complaints that Mr. Shanahan disparaged competitors of Boeing during meetings at the Pentagon with senior national security officials and lawmakers, two United States officials said.
Senator Elizabeth Warren, Democrat of Massachusetts and a member of the Armed Services Committee that would consider Mr. Shanahan’s confirmation if he is nominated, said she alerted the Pentagon’s internal watchdog after hearing that he “may have advocated on behalf of his former employer, Boeing, while serving as the acting defense secretary.”
“The American people should be able to trust that government officials are working for them — not for big defense contractors,” Ms. Warren, who is running for president, said in a statement.
While Mr. Mattis resigned in protest of Mr. Trump’s policies — including the surprise announcement that American troops would withdraw from Syria — Mr. Shanahan has held the line for the president.
As the deputy defense secretary, Mr. Shanahan made clear that “we are not the Department of No,” as he told officials after the administration announced plans to create a new stand-alone Space Force at the Pentagon. It has since been moved to the oversight of the Air Force.
Since becoming acting defense secretary, Mr. Shanahan has raised the ire of some lawmakers. Last month, Senator James M. Inhofe, Republican of Oklahoma and the chairman of the Armed Services Committee, said he did not believe Mr. Shanahan shared the “humility” of Mr. Mattis but did not elaborate. Senator Lindsey Graham, Republican of South Carolina, also clashed with Mr. Shanahan last month over the planned troop withdrawal from Syria.
Last week, Mr. Shanahan appeared in front of Mr. Inhofe’s committee to sell the Pentagon’s proposed $750 billion budget for the 2020 fiscal year, which is geared toward combating threats from Russia and China and modernizing a military that has long focused on counterinsurgency conflicts.
Instead, senators accosted him on a range of issues, including Mr. Trump’s move to take billions of dollars from the Defense Department for a border wall, and the Pentagon’s decision to push for weaker standards on chemicals in drinking water.