Some of the world's wealthiest people joined Warren Buffett's Berkshire Hathaway Inc. in backing embattled insurer UnitedHealth Group Inc. in the second quarter.

George Soros's investment firm, the family office of a Swedish packaging dynasty and Michael Platt's BlueCrest Capital Management all increased their stakes in UnitedHealth last quarter, according to 13F filings published Thursday.

UnitedHealth's stock plummeted this year following a string of setbacks, beginning with the fatal shooting of a top executive in New York City in December. Several months later the company slashed its financial outlook amid mounting medical costs, and its chief executive officer abruptly resigned. Meanwhile, the health-care firm is under federal investigation related to Medicare billing practices.

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