(Reuters) -Shares of UnitedHealth Group surged more than 12% in premarket trading on Friday after Warren Buffett’s Berkshire Hathaway revealed a new investment in the health insurer.

Berkshire owned 5.04 million UnitedHealth shares worth about $1.57 billion as of June 30, it said in a U.S. Securities and Exchange Commission filing.

The investment comes at the time when its shares have been battered amid rising health care costs, a U.S. Department of Justice probe related to its billing practices, a cyberattack, and the shooting death of former top executive Brian Thompson in December.

As of last close, the stock remains the worst performer on the blue-chip Dow Jones Industrial Average so far this year, sliding nearly 46%.

Last month, the company projected full-year adjusted earnings pe

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