Why are the Trump dentists under investigation?

President Donald Trump has reportedly fired four of his top dentists as part of a crackdown on the industry, according to The Washington Post.

Dr. David Alexander and Dr. David Dunn have been charged with obstruction of justice, while Dr. Daniel S. Davis has been charged as a co-conspirator, the Post reported.

Trump also reportedly fired Dr. Stephen A. Smith, who had overseen a $2.8 million settlement between the United States and China, according the Post.

Smith has been a prominent critic of Trump and the president, and has called him a “serial liar” who is unfit for office.

Smith also has said he believes Trump should resign.