"UnitedHealth Group is mired in a crisis seemingly without end. Investors are bracing for another big bout of turbulence given reports of the DoJ investigation," said Susannah Streeter, head of money and markets at Hargreaves Lansdown.

The Justice Department is investigating UnitedHealth Group for possible criminal Medicare fraud, people familiar with the matter said. The healthcare-fraud unit of the Justice Department’s criminal division is overseeing the investigation,