Wall Street suffered turbulent trading on Thursday, with analysts pointing to disappointing corporate results and rising political uncertainty.
The S&P 500 index recorded its worst fall in three months, dropping 1.5% to 2,430 points.
All of the stocks on the Dow Jones Industrial Average - which tracks the 30 biggest listed US companies - fell as the index dropped 1.2% to 21,751.
The Nasdaq index also stumbled, shedding nearly 2% to 6,222 points.
Markets had been rising steadily for weeks, leading analysts to describe stocks as over-valued.
Tensions over North Korea unsettled markets earlier this week.
On Thursday, the terror attack in Barcelona and speculation that one of President Trump's key economic advisers might resign further unnerved investors.
"It is uncertainty on the geopolitical front, on the domestic policy front, and as well it seems as if you have valuations that are stretched," said Chad Morganlander of Washington Crossing Advisors.
"So investors are looking for a reason to actually take risk off at this point in time."
Which stocks suffered the most?
Walmart was the biggest faller on the Dow Jones, dropping 1.6% after it reported lower profit margins and said it could miss forecasts for the current quarter.
Other retail also suffered at the start of trading.
L Brands, which owns Victoria's Secret and Henri Bendel, was one of the biggest losers, plunging more than 10% after it said sales for the year would be lower than expected. It later recovered some of those losses to finish down 5%.
Macy's and Kohl's also fell more than 2%.
But Gap shares climbed nearly 6% in after-hours trade, after the firm told analysts its expecting higher sales for the year than previously forecast.
Shares of Cisco, which trades on the Nasdaq, slipped 4%, after the firm said quarterly revenues fell more than 4% year-on-year.
Payrolls processor Automatic Data Processing fell 5.8%, as activist investor William Ackman pushed for changes at the firm.