And despite confusion over where, how, when and if terrorists might strike, the aim of any attack is clear, reports CBS News National Security Correspondent David Martin: mass casualties on a scale greater than Sept. 11.
The intelligence behind this week's terror alert points to a range of targets, from the headquarters of the U.S fleet in the Persian Gulf in Bahrain, to Reagan National Airport in Washington, to Port of Valdez, Alaska — a major petroleum center.
The mode of attack could be a hijacked commercial airliner or a so-called "dirty" radiological bomb. And the threat seems to apply to a period running to the end of January.
The volume and nature of the threats prompted the move to orange or "high" alert on Sunday, triggering extraordinary security around the nation.
The Pentagon is increasing air patrols over major cities and critical installations like dams and power plants.
The military deployed surface-to-air missile systems in the Washington area and was considering locating more anti-aircraft systems in the New York City region, a defense official said, speaking on condition of anonymity.
One official cautioned that most of the reported threats were uncorroborated — some were from only a single informant or communications intercept — and may be unconnected to a larger al Qaeda plot.
But local officials boosted security at many potential targets, including the Port of Valdez, where armed Coast Guard patrol boats were more visible and ship boardings were on the increase.
The warning from the Homeland Security Department on Sunday about a possible al Qaeda attack this holiday season was focused mainly on the threat from foreign commercial airliners, according to U.S. officials.
But some of the measures are focused on the vulnerability of cargo planes. On Tuesday, the Transportation Security Administration said it had increased inspections of cargo companies to ensure compliance with tighter rules enacted last month.
A TSA spokesman said extra law enforcement added to the perimeters of airports in the past three days was specifically put in place to fortify air cargo facilities.
Also Tuesday, new evidence emerged that terrorists may be planning attacks at Western targets in Turkey during the holidays, according to a police document, while Americans and Britons living abroad were warned to keep a low profile.
Britain's top police officer warned people to be vigilant during the holiday season and in Yemen, security around embassies was noticeably tighter.
The most chilling warning came in Turkey, where an internal police intelligence document obtained by The Associated Press on Tuesday cautioned that militants were infiltrating the country to complete preparations for new attacks against Western targets as well as Istanbul's popular Akmerkez shopping mall.
Militants believed to be linked to al Qaeda bombed two Istanbul synagogues in mid-November and later struck the British consulate and the local branch of a London-based bank. Those attacks killed 62 people, including the suicide bombers.
"It is being detected that the preparations for a large-scale third wave of attacks to follow up the suicide bombings has been completed," said the document, which was signed by two senior police officials, including the head of the Istanbul security department.
Westerners were keeping a low profile Wednesday in the Saudi capital of Riyadh.
"Everybody is aware that there is a terrorist threat and Christmas time would be a more active period for the terrorists to do something," said Richard May, manager of the Seder Village compound in Riyadh, which has been named by the U.S. and British embassies as a possible target.
In Bahrain, the U.S. Embassy in Manama warned that it had received information about a possible terrorist attack during the holidays and cautioned Americans to avoid places where Westerners gather and to reduce unnecessary travel.
The Persian Gulf country is home to the U.S. Navy's Fifth Fleet. More than 5,000 Americans live there.
In the past 13 months, al Qaeda-linked groups have staged major terror attacks in Saudi Arabia, Kenya and Turkey. Yemen was the site of the 2000 bombing of the USS Cole in Aden harbor, which killed 17 U.S. sailors.