Near 11:29 a.m. ET, the Dow was down 123 points (-0.71%), the S&P 500 was down 14 points (-0.72%) and the Nasdaq was down 50 points (-1.05%).
The health and tech sectors led declines on the S&P 500. Apple fell nearly 2% after a Bloomberg report that the company may have to stop iPhone sales in Beijing because they violated a Chinese competitor's patent.
It's quadruple witching Friday, when individual stock options, individual stock futures, index options and index futures all expire. Volume typically spikes on days like these as traders make the appropriate changes to their positions.
Stocks on Thursday ended five straight down sessions and made a huge triple-digit comeback; the Dow finished up 92 points, or 0.53%.
The sell-off earlier in the week has been attributed, in part, to investors getting ready for the British referendum next week, during which a vote to leave the EU could emerge as the majority. After the killing of Labor MP Jo Cox on Thursday, campaigning for the referendum was suspended. She was in support of a "Remain" vote.
Also, the Federal Reserve's more cautious stance on the path of interest-rate hikes was a signal to some that it needed more evidence that data match its assessment and outlook on the economy.
Economic data on Friday is light. Housing starts fell 0.3% at a seasonally adjusted annual rate of 1.16 million, a decline that was lower than expected. Building permits rose 0.7% at a rate of 1.138 million.
At 1 p.m. ET, oil driller Baker Hughes will release its weekly count of US oil and gas rigs. Ahead of the data, crude oil rebounded from a steep sell-off Thursday, with West Texas Intermediate crude futures up 2.6% to $47.43 per barrel in New York.