As UnitedHealth Group wrangles with a federal investigation, an executive shuffle and a tanking stock price, a bullish ally has emerged: Warren Buffett. The Eden Prairie-based health giant’s stock price jumped more than 10% before trading opened Friday, following news that Buffet’s Berkshire Hathaway acquired about 5 million shares.

UnitedHealth shares soared after Warren Buffett revealed a stake of 5 million shares, worth about $1.6 billion, in the troubled health insurer. The "Big Short" investor Michael Burry and Appaloosa Management's David Tepper also disclosed sizable stakes in the company.

Berkshire Hathaway revealed four new investments Thursday in steelmaker Nucor, insurer UnitedHealth and two of the nation’s biggest homebuilders.