U.S. officials, noting sharply increased terrorist communications in recent days, acted with urgency to move the alert status from yellow to orange - from "elevated risk" to "high risk."
"Information indicates that extremists abroad are anticipating near-term attacks that they believe will rival - or exceed - the scope and impact of those we experienced in New York, at the Pentagon and in Pennsylvania [more than] two years ago," Ridge said.
Ridge said al-Qaida terrorists remain interested in using aircraft in its attacks.
After consultation with the federal government, Gov. Robert L. Ehrlich Jr. raised the state's Threat Alert System level to orange from yellow yesterday.
No specific threats have been made against locations in Maryland, the governor said in a statement. "I encourage Marylanders to practice increased vigilance while going about their daily activities and preparing for the upcoming holidays."
The new alert, coming at the peak of the holiday season, will affect millions of travelers, subjecting them to tighter security measures at airports and elsewhere, and will trigger more intense surveillance by federal, state and local law enforcement authorities at borders and around vulnerable targets.
Maryland Transportation Authority Police will increase random vehicle inspections at Baltimore-Washington International Airport, using dogs and mirrors to check vehicles for bombs and hazardous chemicals, said Transportation Authority Police Chief Gary W. McLhinney.
Authority police will also increase their searches of trucks at bridges and tunnels, as well as intensify security at the port of Baltimore. "I don't think there's any reason to panic," said Jim Ports, assistant secretary of the Maryland Department of Transportation. "Go to the malls, go shopping, keeping in mind that if you see something suspicious, let us know."
Suspicious activity can be reported to Maryland's terrorism tip line at 1-800-492-TIPS.
A senior U.S. official, speaking on condition of anonymity, said last night that New York, Washington and Los Angeles remain priority targets for the terrorist network.
This official said the decision to increase the threat level came yesterday morning after a series of high-level discussions that began to intensify Friday involving Ridge's office, the Justice Department, the FBI and the White House.
The official said "chatter," or communications, in extremist circles had spiked in the roughly 48 hours leading up to Ridge's announcement. Reports have led the intelligence community to believe the next two weeks are the time of maximum peril, the official said.
The official said international flights into the U.S. remain a top concern, as well as cargo flights.
For at least the past two months, officials have been operating on the assumption, backed by what they say is credible intelligence, that al-Qaida has put off plans to conduct small-scale operations inside the United States to avoid a large security crackdown. Leaders of the terrorist group feared that such a crackdown would make it impossible to attain the real goal: a mass-casualty attack on par with Sept. 11, 2001, according to senior U.S. officials.
The State Department, in a worldwide caution issued yesterday to U.S. citizens overseas, predicted attempts by al-Qaida to mount assaults "more devastating than the Sept. 11 attack, possibly involving nonconventional weapons such as chemical or biological agents."
Ridge emphasized the seriousness with which the United States views the latest intelligence. "The strategic indicators, including al-Qaida's continued desire to carry out attacks against our homeland, are perhaps greater now than at any point since Sept. 11," Ridge said.
Homeland Security officials have worried about the vulnerability of the aviation industry, especially because of its vital role in the U.S. economy. Ridge said al-Qaida was evaluating aviation procedures "both here and abroad to find gaps in our security posture that can be exploited."
Despite greatly increased airport security, there have been highly publicized breaches. This year, a North Carolina college student was able to sneak box cutters and other suspicious material aboard several Southwest Airlines planes. There was also the New York man who stowed away on a cargo airplane by hiding himself in a crate.
Terrorist experts are worried, however, that the greatest threat to the U.S. aviation industry may come from outside the aircraft in the form of portable, shoulder-fired missiles.
Such missiles have been fired at Western aircraft in Iraq and were responsible for the fatal crash of at least one Army helicopter. In Kenya last year, an Israeli airliner narrowly averted disaster after two missiles were launched at it after takeoff.
Homeland Security, while concerned about aviation, is increasing vigilance in other areas as well. Ridge said the federal government would add personnel to bolster security at the nation's borders and ports.
The Bush administration's action yesterday was the fourth time this year it has raised the terror threat advisory to orange from yellow. It was the first time the level has changed since May.
The administration has been under pressure to limit the frequency of high alerts. Such increases add significantly to law enforcement and other costs of state and local governments, which must pay overtime in order to increase security at possible target sites.
"The increasing of the threat level is significant," wrote David Heyman, director of homeland security studies for the Center for Strategic and International Studies, in an e-mail sent by his Washington think tank. "Every time we go to orange, it costs $1 billion a week to put in place enhanced protective measures across the country."
Noting the burden this puts on major cities, Heyman said, "As a result, after the last threat alert in May, the administration indicated that it would be much more conservative raising the levels in the future. This new warning reflects a real concern on behalf of the administration that the threat to Americans today is more serious than in the past."
The Chicago Tribune and Sun staff writers Sara Neufeld and Johnathon E. Briggs contributed to this article. The Chicago Tribune is a Tribune Publishing newspaper.