Updated at 4:20 p.m. ET.
The Dow Jones industrial average lost more than 317 points today, closing at 16,563, wiping out the index's gains for the month of July.
The Nasdaq fell 93 points, closing at 4,369. The Standard & Poor's 500 fell 2 percent to 1,930.
It was the worst daily decline since April and the first monthly drop since January.
The Associated Press reports investors "responded to several weak earnings reports, escalating geopolitical instability and widespread views that stocks had become too expensive. They are also contemplating the likely end of the Federal Reserve's stimulus program this year."
USA Today says: "With the losses, a five-month string of Dow and S&P 500 Index gains will end with July's last trading session."
According to Fortune:
"Contributing to the market's retreat: news late Wednesday that Argentina's credit rating has been downgraded to selective default by Standard & Poor's. The country's second default in 13 years sent its Merval stock index plummeting Thursday afternoon.
"There are also lingering questions over how the latest round of sanctions on Russia issued by the U.S. and Europe will affect the global economy, including the energy markets. Exxon Mobil XOM, which has a multi-billion dollar partnership with Russian company Rosneft, saw its stock drop 4% on Thursday."