U.S. stocks dropped, with the S&P 500 Index falling to a three-week low, as investors wavered amid a pair of coming central-bank events and Britain's vote on European Union membership that have sowed anxiety.

Equities swung between gains and losses before a retreat accelerated as European stocks closed at their lowest in almost four months. A measure of volatility posted the biggest two-day jump since a selloff last August. Raw-material, industrial and technology shares fell the most, losing more than 1.1 percent. LinkedIn Corp. soared 47 percent after Microsoft Corp. said it's buying the company in a deal valued at $26.2 billion. Microsoft sank 2.6 percent.

The S&P 500 fell 0.8 percent to 2,079.14 at 4 p.m. in New York, marking the longest losing streak in a month and the worst three days of losses since Feb. 9.

As evidence of increasing worry, the CBOE Volatility Index surged more than 20 percent Monday, the most since December and the biggest back-to-back jump since Aug. 24, the day last summer's swoon bottomed. A preference for safety was also apparent today as U.S. Treasuries extended a rally to a fifth day, the longest since February.

Recent declines have signaled a shift in sentiment after as much as $3.3 trillion was added to equities since mid-February. Investors are reassessing the rally amid lackluster economic growth and concerns about the potential fallout from a June 23 referendum that will determine Britain's membership in the European Union.

Also keeping investors on edge is the Federal Reserve's monetary policy review on Wednesday, despite no change predicted for interest rates. Based on Fed funds futures prices, traders don't see at least even odds for a rate increase before February. The Bank of Japan also has a policy meeting scheduled this week.