Trading was feverish due to what’s known as quadruple witching — the expiration of all index and equity options and futures contracts — and investors took the opportunity to adjust their portfolios and move into more defensive stocks. Technology shares, led by a sharp drop in semiconductors, suffered the biggest declines.

"A lot of this weakness is due to options activity," said Michael Sheldon, chief market strategist at Spencer Clarke LLC. "And as you look ahead over the next few weeks, there’s a big tug of war between those who see positives in the market and those who don’t. When that happens, you can expect to drift a little lower."

According to preliminary calculations, the Dow Jones industrial average plunged 109.18, or 1.1 percent, to 10,186.60.

Broader stock indicators also fell sharply. The Standard & Poor’s 500 index fell 12.58, or 1.1 percent, to 1,109.74, and the Nasdaq composite index was down 21.97, or 1.1 percent, at 1,940.47.

For the week, the Dow, which had seen over 206 points of gains on Tuesday and Wednesday, lost 53.48, or 0.5 percent, for the week. The S&P 500 lost 10.83, or 1.0 percent, and the Nasdaq fell 44.26, or 2.2 percent, for the week.

The possible capture of Ayman al-Zawahri kept the market on edge, as traders were concerned that his apprehension might lead to retaliatory terrorist strikes.

Analysts said the next jobs report, due in two weeks, could boost shares somewhat, but most analysts agree there’s very little that will move the market much higher in the near future. Stocks have been falling since mid-February on rising investor anxiety over the lack of job creation, a critical factor if the economic recovery is going to gain momentum.

The renewed terrorist activity, including last week’s bombing of trains in Madrid, has exacerbated the market’s uneasiness and raised the specter of a prolonged slump on Wall Street.

"This has been more of a lengthy correction than we might normally see," said Jeff Kleintop, chief investment strategist for PNC Financial Services Group in Philadelphia. "It’s really been dragged out because of the heightened terrorist activity, high oil prices and low job growth. That keeps everybody on the sidelines."

Even first-quarter earnings reports, due next month, may not be enough to shake the market out of the doldrums.