Shares of UnitedHealth Group slipped Thursday after the health care giant said it was under a Department of Justice investigation. The company said it has started complying with both criminal and civil requests from federal investigators and it was cooperating with them. "(UnitedHealth) has a long record of responsible conduct and effective compliance," the company said in a Securities and Exchange Commission filing. Earlier this year, The Wall Street Journal said federal officials had launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare Advantage, or MA, plans. Those are privately run versions of the government's Medicare coverage program mostly for people ages 65 and over. RELATED STORY | AG seeks death penalty for Luigi Mangione in killing of UnitedHealthcare CEO The company's UnitedHealthcare business covers more than 8 million people as the nation's largest provider of Medicare Advantage plans.