The Department of Justice is digging in its heels against UnitedHealth Group, alleging in a second lawsuit the health insurance giant has knowingly scammed the government by inflating the medical codes of its Medicare Advantage members.

But UnitedHealth is not the only Medicare Advantage insurer under the federal microscope — it's just the largest, covering about one-fourth of all people in the program. The DOJ also is investigating Humana, Centene and other insurers for their coding practices.