Warren Buffett, UnitedHealth Group
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Berkshire Hathaway recently filed its second-quarter 13F form, revealing its stock holdings at the end of June.
The investment comes as UnitedHealth faces soaring medical costs, federal investigations, the fallout of the killing of a top executive and a cyberattack last year.
Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.