Billionaire investor Warren Buffett announced that he will retire from the board of the Washington Post Co. when his term ends in May. The 80-year-old is not saying why he’s stepping down, but he has been preparing his investment firm, Berkshire Hathaway — the publishing company’s largest shareholder — for his eventual departure. He stepped down from the board of Coca-Cola in 2006.

Some analysts speculated that while his decision not to seek re-election could be because of his age, it also could be over the controversy surrounding Kaplan University, the Post’s embattled education unit that is being investigated by the federal government.