SAN FRANCISCO (MarketWatch) -- Monster Beverage MNST-1.84% said late Monday its board doubled the company's share repurchase program to $500 million. The move comes as shares of the energy drinks maker have tumbled 21% since its Aug. 8 earnings report. The company also disclosed it recently got a subpoena from a state attorney general regarding its "advertising, marketing, promotion, ingredients, usage and sale of its Monster Energy brand of energy drinks." Monster Energy has been building up cash on its balance sheet and free cash flow has been growing. During 2011, the company repurchased 5 million shares worth $176.4 million on a split-adjusted basis. However, Monster Energy hasn't repurchased any of its common stock since last October. In after-hours trade Monday, the stock climbed 1% to $54.