Shares in the UnitedHealth Group tumbled after it was reported the largest health insurer in the US is under investigation over possible criminal Medicare fraud.The US Department of Justice has been investigating the health insurance giant since last summer, the Wall Street Journal reported, citing unnamed people familiar with the matter.UnitedHealth’s shares dropped 16.5% during early trading in New York on Thursday, deepening a stark market rout. Its stock value has halved since the turn of the year.Earli ...