Stocks fell, then regained much of the lost ground Friday, following news that the FBI is reopening its investigation into Hillary Clinton's use of a private email server while she was secretary of state.

This was the reaction of the S&P 500:

Markets Insider In a letter to Congress, FBI Director James Comey wrote that the investigative team in charge of looking into Clinton's server briefed him on Thursday on new emails it found that might contain classified information.

The email scandal has plagued Clinton's candidacy for president, and this news comes 10 days before the elections in an apparent October surprise.

A win by Donald Trump would be Wall Street's least preferred outcome, according to Liz Ann Sonders, Charles Schwab's chief investment strategist.

"If we were to go into the election thinking, you know, Clinton's up by 10, 15 points, whatever it is, and then we wake up the next morning and find that Trump has won, I think that's the most unsettling outcome," she told Business Insider in a recent interview.

Stock-market volatility jumped, as measured by the Chicago Board Options Exchange's volatility index, or the VIX: The Mexican peso fell. The currency was down by 0.6%, at 18.9520 per dollar, as of 1:17 p.m. ET after earlier being in the green.

Some currency analysts have argued that a stronger peso is indicative of a Clinton win, given Trump's tough talk about immigration and trade with America's southern neighbor.

Markets Insider

Gold rallied by 0.7%, or $9.60 an ounce, to $1,279. The perceived uncertainty around Trump's presidency is considered a boost for the precious metal, which does well when investors are nervous.