Updated 11:37 PM ET
WASHINGTON - U.S. intelligence officials scrambled to nail down information on a possible al Qaeda strike timed to coincide with the 10th anniversary of the Sept. 11 attacks, while New Yorkers and Washingtonians went about their day seemingly undaunted by talk of a new terror threat.
How valid was the threat? Counterterror experts worked to answer that question for residents and visitors in the two cities that took the brunt of the jetliner attacks that killed about 3,000 people at the World Trade Center and the Pentagon. It was the worst terror assault in U.S. history, and Al Qaeda has long dreamed of striking again to mark the anniversary. But it could be weeks before the validity of the threat is known.
Security worker Eric Martinez wore a pin depicting the twin towers on his lapel as he headed to work in lower Manhattan on Friday, unfazed by a report of a credible but unconfirmed terror threat before the 10th anniversary of the Sept. 11 attacks.
He worked downtown then and lived through it. He still works there -- and didn't hesitate to take the subway Friday.
"It's the only way you can get to work. If something's going to happen it's going to happen. You just have to deal with it," he said. "This is the time we live in. If you're going to be afraid, you're just going to stay home."
Once again, New Yorkers dealt with an ominous-sounding report of a possible threat against the city by taking it in stride. To them, the inconveniences of snarled traffic caused by bridge checkpoints and bag searches at train stations have become part of life in New York City.
Late Wednesday, U.S. officials received information about a threat that included details they considered specific: It involved up to three people, either in the U.S. or who were traveling to the country; a plan concocted with the help of al Qaeda leader Ayman al-Zawahri; a car bomb as a possible weapon and New York or Washington as potential targets.
Officials described the information to The Associated Press on condition of anonymity because they were not authorized to discuss the sensitive matters publicly. Counterterror officials were looking for certain names associated with the threat, but it was unclear whether the names were real or fake.
Government officials say at least two of the men suspected are believed to be U.S. citizens or have U.S. traveling documents.
The officials say the information comes from a single source and it has not been confirmed.
The intelligence community regularly receives tips and information of this nature. But the timing of this particular threat had officials especially concerned, because it was the first "active plot" that came to light as the country marked the significant anniversary, a moment that also was significant to al Qaeda, according to information gleaned in May from Osama bin Laden's compound.
Acutely aware of these factors, law enforcement around the country already had increased security measures at airports, nuclear plants, train stations and more in the weeks leading up to Sept. 11. The latest threat, potentially targeting New York or Washington, prompted an even greater security surge in those cities. U.S. embassies and consulates abroad also had boosted their vigilance in preparation for the anniversary.
Residents in Washington, D.C., were also taking the latest threat in stride. Michael McCabe, a U.S. Department of Energy employee, said he had heard about the threat on the radio and television when he woke up Friday.
"There's been so much speculation that something would be planned that it wasn't a surprise, it's certainly not something I was looking forward to but psychologically was prepared for," McCabe said. He added that the news wouldn't change his plans.
"There's nothing that I can be proactive on other than just aware of the surroundings," he said.
New York City Police Commissioner Raymond Kelly said Thursday night that police were increasing security at bridges and tunnels, setting up vehicle checkpoints, doing bomb sweeps of parking garages and towing more illegally parked cars.
District of Columbia Police Chief Cathy Lanier said officers would be working 12-hour shifts for the near future. She said in a written statement that the scheduling changes were "part of our plan" and that "maintaining a certain sense of unpredictability is essential to the success of any security plan."
But the measures were nothing well-prepared law enforcement agencies haven't seen before, New York City Mayor Michael Bloomberg said Friday.
In fact, Bloomberg told "The Early Show" Friday that with all the city invests in police and counterterrorism efforts, New York is one of the "safest place to be."
"We live in a dangerous world, we know that," Bloomberg said. "We also live in a city that is the symbol of freedom, of a lot of things that the terrorists don't like. So we always have an enormous amount of security here. We have the world's best police department. We work very closely with the FBI, and lots of other intelligence organizations.
"The level of security in this city probably makes New York the safest place to be," he said, noting such security measures as surveillance cameras, radiation detectors, and the city's coordination with overseas police departments, the FBI and the Joint Terrorism Task Force.
On Friday morning, the mayor rode the subway down to City Hall to help assure commuters the city was prepared.
"We don't want al Qaeda or any other organization ... to take away the freedoms without firing a shot," he said after getting off the train near the Brooklyn Bridge. Bloomberg urged New Yorkers to just "go back to work. And leave it to the professionals."
Police planned a show of force at Grand Central Terminal, Pennsylvania Station and the Times Square subway station because of a previously planned counterterror drill with rail agencies.
Authorities were stopping vehicles at the 59th Street bridge, which connects Manhattan to Queens, causing a major backup. The Brooklyn Bridge was down to one lane, and checkpoints were set up near Times Square and in other Midtown locations.
At Penn Station, transit police in helmets and bulletproof vests and carrying assault rifles watched the crowds.
Officials were swabbing passengers' bags near an escalator to the train platforms, and police searched the bags of passengers at the entrance to a subway station. National Guard troops in camouflaged fatigues moved among the throng, eyeing packages.
Roseanne Lee, 64, said her taxi was stopped twice at police checkpoints on its way from the Upper East Side to Penn Station. Police looked in the windows of the cab but did not question her or the driver, she said. At one checkpoint, police were searching a moving van, she said.
The delays turned a 15-minute ride into a 35-minute one and cost her $21 instead of the usual $12.
"But I don't care," Lee said. "It's better to be safe. You can't stop doing what you're doing because of these threats. You just have to be careful."
Gail Murray, an administrative assistant who works in Manhattan, took the threat in stride as she listened to Long Island Rail Road announcements aboard a train heading from Queens Village to Penn Station.
"I thought, 'Here we go again,'" she said. "That's all just part of living in New York City."
She said whether or not she was worried, she would have to take mass transit.
"I don't have the luxury of working from home," she said.
Police tours were extended, effectively increasing the strength of the patrol force, and the department prepared to respond to an increase in calls of suspicious packages. They also added more police vehicles with license plate readers.
"There will be increased focus on tunnels and bridges and infrastructure in general, as well as landmark locations, houses of worship and government buildings," Kelly said.
The Port Authority of New York and New Jersey, which owns the World Trade Center site, was also at a heightened state of alert. Spokesman Steve Coleman said there would be increased vehicle checks at all crossings, increased police presence and increased bag checks at the authority's airports, bus and rail terminals.
City officials said there also would be a lot done behind the scenes, in places that New Yorkers wouldn't even notice.
Kelly stressed the most important thing to do was to go on with life as usual.
Many New Yorkers were doing just that. Dressed in jeans and a Teamsters T-shirt, Michael Murphy of Seaford, didn't have terror threats on his mind as he headed to work at the armory at 26th and Lexington Avenue, where he was helping to stage shows for Fashion Week.
"Like they said last night, we have the greatest police department in the world," said Murray, 49. "I'm confident they'll do the job."
There were similar scenes in the nation's capital Friday.
Cheryl Francis, of Chantilly, Va., said she commutes over the Roosevelt bridge into Washington every day and didn't plan to change her habits. Francis said she had been at work in Washington on Sept. 11, 2001, and believes the country is more aware and alert now. She said people need to continue living their lives, but also be aware of what is going on around them.
"It's almost like sleeping with one eye open," she said.
On Thursday night, dozens of travelers were lined up at Union Station to take Amtrak and commuter trains home from Washington. There was no visible increase in security for those boarding trains after news of a security threat, though some passengers said they had noticed police officers and dogs earlier.
On Friday, 31-year-old Janaya Williams was waiting for a train to Philadelphia at Union Station. She hadn't even heard of the threat yet.
"I watch SpongeBob all day," said Williams, the mother of a 2- and 4-year-old. She said she prays for her family daily, and that's all she can really do.
"I figure if it's going to happen it's going to happen," she said of the possibility of a terror attack.
Maria Rothman was more frightened. She had changed her travel plans to take an earlier train at 6:30 a.m., hoping it would be a less inviting terror target. She wound up oversleeping and was waiting for a 10:20 train to Philadelphia. From there, she was headed about 45 minutes outside the city to be with her fiance. It can be difficult to go about business as usual, Rothman said.
"I embrace as a concept you got to go ahead and live your life, except that it's hard," she said.