Investing.com-- Unitedhealth Group (NYSE: UNH ) is being investigated by the Department of Justice over possible criminal fraud involving Medicare, the Wall Street Journal reported on Wednesday.

UnitedHealth shares slid 6.5% to a near five-year low in aftermarket trade, following the report.

The DOJ’s healthcare-fraud unit has been actively investigating the insurer since at least last summer, the WSJ report said, citing people familiar with the matter.

The exact nature of the criminal allegations are unclear, and the investigation is looking into UnitedHealth’s Medicare Advantage business practices.

Reports of the DOJ probe come as UnitedHealth grapples with allegations of potential antitrust violations, as well as a civil investigation into its Medicare billing practices.

The company abruptly replaced its CEO Andrew Witty with Chairman and former CEO Stephen Hemsley this week, as it also faces a steep decline in its stock price this year.

This comes against the backdrop of potential reductions in federal healthcare spending, which has been a key driver of UnitedHealth’s earnings. President Donald Trump is seeking to sharply lower drug prices under Medicare.