U.S. equities closed mostly lower on Friday after the Federal Bureau of Investigation announced it is investigating new emails related to Democratic nominee Hillary Clinton.

The Dow Jones industrial average ended about 10 points lower after trading 74.71 points lower following the announcement. The index was trading about 75 points higher before the new probe was announced.

"I sit next to one of the traders here and he was telling me he'd never seen the Dow fall so quickly," said Art Hogan, chief market strategist at Wunderlich Securities. "There's a lot more to this, but we won't know how much until we get more information."

"Regardless of the fundamental news we get next week, earnings or data, this may be the story for the next 10 days," he said.

The S&P 500 closed about 0.3 percent lower, and was also trading higher before the FBI's probe was announced. The Nasdaq composite ended about half a percent, off its session lows.

"She had a pretty big lead. Now this will certainly take a chunk of that, but most investors still think she'll win on November 8," said Jeremy Klein, chief market strategist at FBN Securities.

Investors were also watching the 2,130 level on the S&P, a level pointed out by DoubleLine Capital CEO Jeffrey Gundlach last week. He said closing below that level would be concerning for the stock market. S&P closed around 2,126 on Friday.

"The market has been very sanguine in pricing in a victory for Hillary Clinton. If something happened that threatened that outcome, then that would be hurtful for the market given the uncertainty surrounding Donald Trump's fiscal policy," said Phil Orlando, chief equity strategist at Federated Investors.

Clinton's lead over Trump had narrowed even before news of the new probe had broken, according to data from RealClearPolitics.

Art Cashin, UBS director of floor operations at the NYSE, said the market fell in part "delayed reaction to oil and a little bit to the Hilary thing because that's a new surprise, you only have 11 days to go to the election. How can the FBI reopen the case and resolve the thing in 11 days? That's what the market is worried about."



"The weekend talk shows will be important. The big questions will be how can they complete an investigation in just nine days. IT certainly means that probably before the inauguration if the republicans hold the house, there will be an investigation that lasts four years," said Cashin.

U.S. Treasury yields hit their session lows following the announcement. The two-year note yield lower traded around 0.85 percent and the benchmark 10-year yield held near 1.84 percent. The benchmark yield has been rising steadily over the past three months.

Earlier, the three major indexes had drifted higher following the release of better-than-expected U.S. GDP data, while investors continued to parse through a slew of corporate earnings results.

Dow Jones industrial average intraday chart