Updated from 4:02 p.m. EDT

The major stock indices tumbled Thursday to their lowest closing levels since late May, as investors already sifting through further profit warnings from the tech sector heard a foreboding message about terrorism in the U.S. that sent oil back up above the $40-a-barrel mark.

The Dow closed down 68.73 points, or 0.67%, to 10,171.56; the S&P 500 fell 9.23 points, or 0.83%, to 1109.10; and the Nasdaq lost 30.76 points, or 1.56%, to 1935.32. The 10-year Treasury note traded up 1/32 in price to yield 4.47%, while the dollar was higher against the yen and lower against the euro.

In moderate summer trading, volume approached 1.4 billion shares on the New York Stock Exchange, where decliners outnumbered advancers by about 2 to 1. On the Nasdaq, almost 1.8 billion shares changed hands, and decliners held about a 7-to-2 majority.

Homeland Security Secretary Tom Ridge said in a news conference that American intelligence analysts have credible information that al Qaeda terrorists are planning another attack in the U.S., but he had no specifics and said there was no reason to raise the terrorist-threat level for now.

"The latest White House announcement about terrorism certainly set the stage for oil prices to rally, and that is causing further pressure in the market," said Peter Cardillo, chief market analyst with S.W. Bach & Co. "There's a host of woes out there now that will likely take us down to the bottom of our trading range."

Crude oil prices rose for the fourth session in six on Thursday, closing above $40 a barrel for the first time in five weeks. The benchmark U.S. crude gained $1.12, or 3%, to $40.20, after closing lower Wednesday. Gasoline futures rose almost 5 cents, or 3.7%, to $1.32 a gallon.

Adding to pressure on stocks, reports of violence in Iraq continued. Insurgents fired mortar rounds at an Iraqi National Guard headquarters in the city of Samarra on Thursday, killing four U.S. soldiers and one Iraqi guardsman, according to The Associated Press. A fifth U.S. soldier was unaccounted for and 20 people were wounded.