Stocks extended heavy losses Thursday through to the close in what is shaping up to be Wall Street's worst week in more than two months. The S&P 500 and Dow Jones Industrial Average closed with losses of more than 1% apiece. GoPro (GPRO) closed below its IPO price of $24 a share for the first time since floating in mid-2014. The company has suffered big losses in recent weeks since its third quarter showed disappointing demand for its Hero4 Session camera. Crude oil closed at its lowest level since the end of August after a greater-than-expected increase in weekly inventories. U.S. crude stocks added just over four million barrels in the past week, compared to an expected increase of just under three million. The energy sector was one of the worst market performers on Thursday.