Stocks fell victim to a late afternoon swoon after trading little changed for most of the day, sending benchmark indexes to session lows before recovering slightly.
The S&P 500 and Dow ended lower for the second day in a row, while the Nasdaq has slipped for four straight days for the first time since August.
The drop occurred in light volume and didn't appear to be based on any news event; one trader says stocks are "just bouncing around in a range... We're in this period where we're waiting to see what the next big thing is going to be."
Commodities traders remain worried about a slowdown in China's economy: Crude oil futures fell 1.1% to settle barely over $100/barrel, their lowest level in a month, and copper prices plunged to four-year lows.
Energy firms led the slide in stocks; of note was the huge intraday reversal in fuel cell stocks, which had been bid up in some cases to multiyear highs.
The yield on 10-year Treasurys fell 1.5 bps to 2.768%.