A sales warning from National Semiconductor Corp., which was downgraded by brokerage firm Smith Barney, added to the pressure on the technology sector. The sell-off erased much of the gains the Nasdaq composite index had the previous session, and led the rest of the market lower.

Volatility in oil prices, spurred by concerns about tightening supply, also contributed to the decline. Crude futures fell after Saudi Arabia signaled it could raise its production output if necessary, but resumed their climb later, closing up 28 cents at $44.80 — 4 cents off their record.

"We keep coming back to the same concern: crude oil prices," said Ken Tower, chief market strategist for Schwab's CyberTrader. "Until there is some sign that crude oil is starting to come down O and stabilize, I think everybody is going to remain very uncertain about how much of a slow down in growth we will have."

According to preliminary results, the Dow Jones industrial average closed down 6.35, or 0.1 percent, at 9,938.32.

The tech-dominated Nasdaq fell 26.28, or 1.4 percent, to 1,782.42. The Standard & Poor's 500 index shed 3.25, or 0.3 percent, to 1,075.79.

Investors had a muted reaction to yesterday's much-anticipated rate hike, which brings the federal funds rate to 1.5 percent, focusing instead on the Federal Reserve's upbeat assessment of the economy. The Fed's Open Market Committee blamed much of the recent slowdown on high energy prices, and said the economy was poised to improve.

The suggestion that more rate hikes lie ahead gave Wall Street some pause, however. Most analysts agree that the market's fundamentals remain strong, but a long list of unknowns continue to worry investors.

"We feel good about the broader economy, on the back of the Fed release ... and are confident going forward. And corporate earnings are all solid," said David Hegarty, head trader at Commerzbank Securities. "But there are a number of things that are just keeping the pressure on us: energy, terrorism, increasing interest rates and a tight presidential race."

Cisco's after-the-bell report yesterday's did little to quell fears about the second half of the year. The networking company beat earnings expectations, but its cautious outlook on corporate technology spending rattled investors. Cisco shed 11 percent, or $2.17, to $18.29.

National Semiconductor sank 14 percent, or $2.20, to $13.50, after it said revenues were likely to miss forecasts because of a drop-off in orders. Its remarks raised alarm among analysts about prospects for the chip sector.