Shares of UnitedHealth Group surged before the market open Friday as Warren Buffett’s Berkshire Hathaway disclosed that it recently purchased shares of the beleaguered insurer.

Berkshire Hathaway bought around 5 million shares of UnitedHealth last quarter, according to a regulatory filing. The stake was valued at about $1.57 billion.

Buffett plans to retire as CEO at the end of the year after six decades of building Berkshire Hathaway. Many investors comb through Berkshire’s filings every quarter because they like to follow Buffett’s moves.

The filing doesn’t make clear who at Berkshire handled the investment. Besides Buffett, Ted Weschler and Todd Combs also pick stocks, but they generally handle smaller portfolios and Combs also serves as Geico’s CEO.

Besides stocks, Berkshire owns dozens of companies in a variety of industries including Geico insurance, BNSF railroad, several major utilities and an assortment of manufacturing and retail companies. The Omaha, Nebraska-based company’s holdings include many well-known brands like See’s Candy and Dairy Queen.

UnitedHealth has been dealing with a series of difficulties. Last month the company said that it was cooperating with federal criminal and civil investigations involving its market-leading Medicare business.

The health care giant said at the time that it had contacted the Department of Justice after reviewing media reports about investigations into certain elements of its business.

Earlier this year, The Wall Street Journal said federal officials had launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare Advantage, or MA, plans. Those are privately run versions of the government’s Medicare coverage program mostly for people ages 65 and over.

The company’s UnitedHealthcare business covers more than 8 million people as the nation’s largest provider of Medicare Advantage plans. The business has been under pressure in recent quarters due to rising care use and rate cuts.

The Journal said in February, citing anonymous sources, that the probe focused on billing practices in recent months.

The paper has since said that a federal criminal health care-fraud unit was investigating how the company used doctors and nurses to gather diagnoses that bolster payments.

UnitedHealth Group Inc. runs one of the nation’s largest health insurance and pharmacy benefits management businesses. It also operates a growing Optum business that provides care and technology support.

UnitedHealth’s stock has mostly shed value since December, when UnitedHealthcare CEO Brian Thompson was fatally shot in midtown Manhattan on his way to the company’s annual investor meeting. A suspect, Luigi Mangione, has been charged in connection with the shooting.

In April, shares plunged some more after the company cut its forecast due to a spike in health care use. A month later, former CEO Andrew Witty resigned, and the company withdrew its forecast entirely, saying that medical costs from new Medicare Advantage members were higher than expected.

UnitedHealth’s stock jumped more than 12% in premarket trading on Friday. Still, the stock has lost roughly half its value in the past year.