It keeps getting worse for UnitedHealth Group Inc. investors, as a report of a criminal investigation into its Medicare practices sent the stock diving again in early Thursday trading.

The healthcare giant is under investigation for possible Medicare fraud, "The Wall Street Journal" had reported.

Federal investigators are investigating a major health insurer for possible criminal Medicare fraud, people familiar with the matter told The Wall Street Journal on Wednesday.Health care fraud investigators at the Justice Department have been probing UnitedHealth Group since at least the summer of