Patrick Shanahan's expected promotion to permanently head the Defense Department has stalled amid an ethics investigation and a series of unimpressive public performances, according to four people with knowledge of internal White House discussions.

Shanahan, who has served as the Pentagon's acting chief since Jim Mattis resigned in December, has also been hampered by the deadly crashes of two airliners manufactured by his former employer, Boeing -- the company that's also at the heart of his ethics problems.