The information has been gleaned from sources as diverse as al-Qaida mastermind Khalid Shaikh Mohammed and low-level terror network members in Saudi Arabia and has led U.S. officials to quickly adapt security procedures several times in recent months, the officials said.

For instance, U.S. officials have been discreetly working with their counterparts in Canada and Mexico on improved security measures after intelligence indicated al-Qaida might use in an attack an international airliner that simply passes over U.S. soil, the officials said.

"The information clearly shows they care about getting ahold of airplanes with large fuel supplies in areas with lots of people, and to do it in a way that comes in below our radar screen," said one senior U.S. official with access to intelligence, speaking only on condition of anonymity.

Bush administration officials said an improving apparatus that directs credible threat information quickly from FBI (news - web sites) and CIA (news - web sites) agents in the field to intelligence analysts, and then to homeland security and transportation officials for action, has resulted in quick adaptations of the security net.

One example came late this summer when the State and Homeland Security departments abruptly ended two long-standing programs that had allowed foreign travelers to stay in U.S. airports without visas during interim stops while awaiting flights to other countries.

The change was made with little fanfare in August based on intelligence from Mohammed and other prisoners that al-Qaida was looking to exploit the program to gain access to fully fueled international jetliners, officials said.

"Recent specific intelligence indicates that terrorist groups have been planning to exploit these transit programs to gain access to the U.S. or U.S. airspace without going through the consular screening process," said a Homeland Security advisory to law enforcement and airlines at the time.

The change was made quickly in consultation with U.S. air carriers even though it inconvenienced large numbers of international business travelers, most from U.S.-friendly countries, who had become used to being allowed to stop temporarily in the United States and remain in the Customs area of airports without having to obtain a U.S. visa.

Officials said they were studying ways to re-institute the two programs in a more limited manner to return some flexibility to international travelers who simply have a stopover on U.S. soil in places like Miami, New York City and Los Angeles.

Government and private security experts said al-Qaida's continued fascination with aviation as a method for attack may seem surprising at first blush since it has had one spectacular success during an era of loose security and now faces a more formidable safety net that includes steel cockpit doors, air marshals, pilots with guns, enormous preflight screening and a suspicious traveling public that is quick to react to perceived threats.

"You would think there are a lot of softer targets out there that wouldn't take as much work," said Douglas Laird, a former Northwest Airlines security chief who now advises clients on aviation security. "What baffles me is I just don't know what they (al-Qaida) would do to make it happen now."

But Laird said an aviation attack continues to have appeal to the terrorist mind. "The airlines are always going to be a good avenue because it strikes the fear of God in the public to have an airliner sabotaged. It generates lots of coverage," he said.

U.S. officials said debriefings of terrorists show al-Qaida continues to value an attack via aviation because its leaders believe such an attack would have both dramatic effects on America's economic superiority and because it would strike at an important symbol of Western freedom.

They said the information that led to Sunday's decision to raise the U.S. terror alert to its second-highest level included some information related to airlines, particularly international flights that might enter U.S. air space, although they declined to be more specific.

Homeland Security officials quickly reacted by stepping up security sweeps at airports, adding more air marshals to flights and increasing the number of military flyovers over major cities like Los Angeles, Washington and New York for the holidays.

Despite such marked improvements in security, some in Congress still see holes that could be exploited.

Rep. Edward Markey, D-Mass., a member of the House Homeland Security Committee, on Monday called for the Bush administration to step up screening of cargo that is transported on commercial jets, calling it "the most obvious remaining hole in our aviation security system."