The Pentagon has launched an inquiry into acting US Defence Secretary Patrick Shanahan for alleged favouritism to his ex-employer, Boeing.
The Defence Department's inspector general will look into the matter following a complaint from a watchdog group.
Mr Shanahan is accused of frequently praising Boeing in meetings about government contracts and acquisitions.
Mr Shanahan, who denies any wrongdoing, spent 30 years at Boeing.
He rose through the ranks to become a senior executive at the world's biggest planemaker.
Last week Citizens for Responsibility and Ethics in Washington filed a complaint with the Pentagon inspector general about Mr Shanahan.
The complaint said he had appeared to violate ethical rules by "promoting Boeing in the scope of his official duties... and disparaging the company's competitors to his subordinates".
Dwrena Allen, a spokeswoman for the inspector general, said in a statement on Wednesday: "The Department of Defense Office of Inspector General has decided to investigate complaints we recently received that Acting Secretary Patrick Shanahan allegedly took actions to promote his former employer, Boeing, and disparage its competitors."
Mr Shanahan said last week during a Senate Armed Services Committee hearing that he would support an investigation by the inspector general.
Democratic Senator Elizabeth Warren, a member of the panel, said she had led calls for the inquiry.
She tweeted on Wednesday: "Government officials should work for the people - not big defence contractors."
The inquiry casts a shadow over Mr Shanahan as the White House considers whether to formally nominate him to fill the defence secretary post left vacant by Jim Mattis, who stepped down in December.
Boeing is already under pressure after the deadly crash of one of its 737 Max 8 passenger jets in Ethiopia last week.
The FBI is reported to be assisting the investigation into safety issues surrounding the Boeing airliner.
Another of the passenger planes crashed in Indonesia last October, also killing everyone on board.
According to the Seattle Times, the FBI is investigating the process that led to the aircraft getting its safety certification.
The US Department of Justice has refused to comment on claims that it has been looking at the Federal Aviation Administration's oversight of Boeing.