"In terms of the market and its possible response on Wednesday, today's reaction to the FBI news is obvious evidence that market participants have wrapped up the world in a nice and easy box," said Peter Boockvar, chief market analyst at The Lindsey Group.
"Hillary wins, good for stocks. Trump wins, bad for stocks. But, anyone who thinks deeper than this knows that the response is going to be much more nuanced. Ask any owner of a healthcare, financial or defense stock," he said in a note.
Naeem Aslam, chief market analyst at Think Markets, said in a note investors should also watch out for any surprises. "Donald Trump's victory will not only bring the knee jerk reaction for the market, but we are also concerned about the geopolitical uncertainty," he said.
In a letter sent to Congress on Sunday, FBI Director James Comey said the bureau had "not changed its conclusions," reached in July, on Clinton's private email server. This new investigation, which came to light on October 28, kept financial markets across the globe on edge.
Entering Monday, the S&P had fallen more than 2 percent since the new probe was announced. The Mexican peso, considered a proxy trade for the U.S. election — which will be held on Tuesday — had fallen as much as 3.89 percent since October 28. Since then, Clinton's lead in the polls over Trump has narrowed significantly, according to data from RealClearPolitics.
"From everything I've read, it seems like Clinton's winning by about 2 percent, ... but there is a concern out there that the polls are not correct, and that they underestimate the number of Trump supporters," said Thomas Wilson, senior investment manager at Brinker Capital.
But the Mexican currency popped around 2 percent against the greenback Monday to trade near 18.653. Mexican stocks also ripped higher, as the iShares MSCI Mexico Capped ETF (EWW) gained 5.4 percent, on posting its largest one-day percentage increase since 2011.
The U.S. dollar also rose broadly against a basket of currencies, with the euro near $1.104 and the yen around 104.5.
Other asset classes also saw significant moves, as investors unloaded previous risk-off positions. U.S. Treasurys fell, with the two-year note yield near 0.81 percent and the benchmark 10-year yield around 1.82 percent.
Gold futures for December delivery also fell, settling $25.10 lower at $1,279.40 per ounce. Meanwhile, the CBOE Volatility Index (VIX), widely considered the best gauge of fear in the market, fell 16.2 percent lower, near 18.87, and snapped a nine-day winning streak.
S&P (blue) vs gold futures (green) intradaySource: FactSet
Separately, in an interview with CNBC's "Squawk Box," Appaloosa Management founder David Tepper said Monday he has become more bullish on stocks in the past few weeks after generally being "pretty cautious on the market, not outright bearish."
In oil markets, U.S. crude gained 1.86 percent to settle at $44.89 per barrel, boosted by a commitment from OPEC to stick to a deal to cut output, but prices remained more than $7 below last month's high due to persistent doubts over the feasibility of the group's plan.