Federal investigators are looking into how Boeing's 737 Max plane was certified to fly.

The FBI is also joining the Department of Transportation's inquiry, The Seattle Times reported.

The FBI is joining the Department of Transportation's investigation into the Boeing 737 Max plane, The Seattle Times reported on Wednesday.

People familiar with the matter told reporter Steve Miletich that the investigation is focused on how the plane, which has crashed twice in the past five months, was certified to fly. Department of Transportation Secretary Elaine Chao formally requested an audit of the Federal Aviation Administration's certification for the process in a letter sent to the agency on Tuesday.

The FBI did not immediately respond to a request for comment.

Separately, the Pentagon's inspector general said on Wednesday that it will investigate a watchdog group's allegations that acting Defense Secretary Patrick Shanahan has used his office to promote his former employer, Boeing.

Read the full Seattle Times report here.

More on the investigations into Boeing and its 737 Max plane: