CBS News Correspondent Jim Stewart reports the tone of the messages intercepted by U.S. intelligence agencies, according to one official, was that "something spectacular will happen in the U.S. before the end of the year."
Another official said the intelligence suggested that senior al Qaeda leaders, perhaps even Osama bin Laden himself, had signed off on the attack.
Much of the latest threat data still points to U.S. interests overseas, with areas like Saudi Arabia, Turkey and East Africa deemed particularly vulnerable.
But analysts agree that it is the United States that al Qaeda most wants to hit.
"The group is committed to attacking America on its own soil," said Rand Corporation analyst Steven Simon. "That's the big prize. That's what they want to achieve."
The intercepted communications indicate al Qaeda may plan to use foreign commercial flights bound for the United States because security at foreign airports is considering more lax. In response to the threat, the U.S. has changed some aviation security procedures, reports CBS News White House Correspondent Peter Maer.
Specifically mentioned as targets were New York, Washington and Los Angeles, as well as large public celebrations, which some government analysts believe suggests events like parades and football games.
But there is no firm indication on the location or type of target. The Los Angeles Times reports al Qaeda actually may be plotting a series of attacks, hitting not only large cities but also remote but important areas — like Valdez, Alaska, where key oil facilities are located.
Officials are said to be frustrated by the variety of signals the terrorist "chatter" is sending.
Officials acknowledge they're worried that by raising the code to orange too often, they will wear out its effectiveness, but insist this time it was worthwhile, and that they'll probably keep it in effect into the new year.
The Times reports that unlike on previous occasions, official were unanimous that the alert level must be raised this time.
"I have never seen the national security leadership as tense and anxious as they are right now," an official told the Times.
"There are a number of credible sources that suggest the possibility of attacks against the homeland around the holiday season and beyond," White House spokesman Scott McClellan on Monday.
"There's still information that indicates that terrorists abroad are anticipating attacks that they believe will rival or exceed the scope and impact of those we experienced on September 11th," McClellan said.
The information has been gleaned from sources as diverse as al Qaeda 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 Qaeda 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 a hold 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.
U.S. officials said debriefings of terrorists show al Qaeda 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.
Reacting to the terror alert, already tight security has been fortified at transportation and energy facilities nationwide.
President Bush said the government was doing its best to protect the country and advised citizens "to go about their lives."
"But as they do so, they need to know that governments at all levels are working as hard as we possibly can to protect the American citizens," Mr. Homeland Security officials quickly reacted by adding more air marshals to flights and increasing the number of military flyovers over major cities for the holidays.
Bomb-sniffing canine units were added at Cincinnati/Northern Kentucky International Airport, maritime patrols were augmented near power plants that border Lake Erie, and more officers than usual were activated along the U.S.-Canadian border.
Similar defenses have been increased around key bridges, tunnels, seaports and landmarks, as well as chemical facilities and other places that may be vulnerable to attack.
Bush said in a statement.
To defend against another airborne attack, baggage and passenger screeners are working overtime at airports, and their managers have come out from behind the scenes to work the front lines, the TSA said.
The Transportation Security Administration also reminded travelers that it needs help, broadcasting messages over loudspeakers at airports and train stations that urge Americans to report any suspicious activity or unattended luggage.
The TSA's Mark Hatfield told the CBS News Early Show that travelers ought to add up to a half-hour to their arrival at the airport.
"In addition to that, people can do a lot to prepare to get through security more quickly. Simply things like making sure you don't have prohibitive items with you," he said, like tools and pocket knives." Don't wrap things before you get to the airport."
Elsewhere, London's top police officer warned Britons Tuesday to be vigilant against terrorism over the Christmas period, but said he had no specific intelligence about a planned attack.