Many investors had believed Sokol was in line to one day succeed Buffett as Berkshire's CEO partly because Buffett regularly praised him publicly, but Buffett said later that Sokol's departure had no effect on Berkshire's succession planning.
Buffett did not respond Friday to a request for comment about Sokol.
At the time of his resignation, Sokol was chairman of Des Moines, Iowa-based MidAmerican Energy — one of Berkshire's biggest subsidiaries — and CEO of NetJets. He was also chairman of Johns Manville and represented Berkshire on the board of Chinese car and battery maker BYD.
Since leaving Berkshire Hathaway, Sokol has been managing his own personal investments. Primarily, he has been steadily buying shares of Middleburg Financial, which operates bank branches in the suburbs of Washington, D.C.
Sokol holds 1.7 million shares of Middleburg's stock, giving him control of nearly 25 percent of the banking firm.
Several months after Sokol resigned, Berkshire did acquire Lubrizol. That chemical firm joined a group of more than 80 companies Berkshire owns outright, including Geico insurance, General Reinsurance, BNSF railroad, Acme Brick, Helzberg Diamonds and many others.
Berkshire also holds major investments in Wells Fargo, IBM, American Express, Washington Post, Coca-Cola and several companies.
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Berkshire Hathaway Inc.: www.berkshirehathaway.com