The long-running mystery around Warren Buffett’s latest stealth investment is finally over. In Berkshire Hathaway’s (NYSE: BRK.B) newest 13F filing, the Oracle of Omaha revealed a fresh stake in UnitedHealth Group Incorporated (NYSE: UNH).

Shares of UnitedHealth Group surged nearly 14% on Friday after billionaire Warren Buffett's Berkshire Hathaway bought 5 million shares of the company, providing a shot in the arm for investors who think the health conglomerate will turn around under its new CEO.

The company may be under criminal investigation for alleged irregularities in its Medicare business—but it's been touched by the Warren Buffett effect.

Warren Buffett's Berkshire Hathaway built a $1.6 billion stake in UnitedHealth last quarter, marking one of its last wagers with the investor as CEO.

B stake in UnitedHealth Group Incorporated signals a market shift. Click for why I have now upgraded UNH stock to Buy.

UnitedHealth Group Inc (NYSE: UNH) has transformed from Wall Street darling to crisis headline - yet ironically, a who's-who of investors are doubling down even as volatility reigns. Legendary investors are betting on UNH's recovery and analysts see potential upside despite recent struggles.

Berkshire Hathaway recently filed its second-quarter 13F form, revealing its stock holdings at the end of June.