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.

Berkshire Hathaway revealed four new investments Thursday in steelmaker Nucor, insurer UnitedHealth and two of the nation’s biggest homebuilders — Lennar and DR Horton — but only one of the investments was big enough to ensure that legendary investor Warren Buffett may have handled them.

Buffett's $1.57B bet on UNH sparks a 12% surge, lifting peers despite sector headwinds and recent struggles at the insurer.

UnitedHealth was a star stock late last week, when it was revealed that Warren Buffett's Berkshire Hathaway had taken a $1.6 billion stake in the company's equity. On Monday, lingering positive sentiment on UnitedHealth was boosted by that price target bump.

On Friday, high-profile investors Michael Burry and Warren Buffett took positions in UnitedHealth Group Inc. (NYSE:UNH). Warren Buffett's fund bought little more than 5 million shares, and Burry disclosed in 13F filings that he purchased call contracts against 350,

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.

Warren Buffett's mystery stock purchase has sparked speculation and a frenzy. The only certain clue is that it's not a bank, insurer, or consumer products company.