Many worry the platform was hacked; others claim to have lost thousands of dollars.

European Commission President: EU is prepared to respond by targeting imports of Harley-Davidson Inc. motorbikes, Levi Strauss & Co. jeans, and U.S. bourbon whiskey.

Companies are leery about their longer-term prospects, which means many will continue to hoard cash or distribute to shareholders, rather than making capital investments.

"That the S&P is down this much even as yields are falling tells me that there is probably still a lot of fear about how sustainable the bounce is."