WASHINGTON (AP) — Undaunted by talk of a new terror threat, New Yorkers and Washingtonians wove among police armed with assault rifles and waited with varying degrees of patience at security checkpoints Friday while intelligence officials scrambled to nail down information on a possible al-Qaida strike timed to coincide with the 10th anniversary of 9/11.

How valid was the threat? Counterterror experts worked to answer that question before Sunday 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 the nation’s history, and Al-Qaida has long dreamed of striking again to mark the anniversary. But it could be weeks before the answer is clear.

Security worker Eric Martinez wore a pin depicting the twin towers on his lapel as he headed to work in lower Manhattan on Friday. He worked downtown 10 years ago and lived through it all. He still works there and said Friday, “If you’re going to be afraid, you’re just going to stay home.”

Mayor Michael Bloomberg, too, made a point of taking the subway to City Hall.

People should take “all necessary precautions” this weekend, said President Barack Obama, who was briefed on the threat Friday morning in Washington. He still plans to travel to New York on Sunday to mark the 10th anniversary with stops that day at the Pentagon and Shanksville, Pa., too.

Washington commuters were well aware of the terror talk.

Cheryl Francis, of Chantilly, Va., said she travels over the Roosevelt bridge into Washington every day and doesn’t plan to change her habits. Francis, who was in Washington on Sept. 11, 2001, said a decade later the country is more aware and alert.

“It’s almost like sleeping with one eye open,” she said, but she added that people need to continue living their lives.

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-Qaida 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 only on condition of anonymity because they were not authorized to publicly discuss the sensitive matters. Counterterrorism officials were looking for certain names associated with the threat, but it was unclear whether the names were real or fake.

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 was also significant to al-Qaida, according to information gleaned in May from Osama bin Laden’s compound.

The U.S. government has long known that terrorists see the 10th anniversary of 9/11 and other uniquely American dates as opportunities to strike. Officials have also been concerned that some may see this anniversary as an opportunity to avenge bin Laden’s death.

Britain, meanwhile, warned its citizens who are traveling to the U.S. that there was a potential for new terror attacks that could include “places frequented by expatriates and foreign travelers.”

Acutely aware of these factors, law enforcement around the country had already 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 had also boosted their vigilance in preparation for the anniversary.

At Penn Station in New York, transit authority police carried assault rifles and wore helmets and bullet proof vests as they watched crowds of commuters. Police searched passengers’ bags as they entered the subway, and National Guard troops in camouflage fatigues moved among riders, eyeing packages.

Retired kindergarten teacher Roseanne Lee was in town from Islip, N.Y., to visit her son and 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 ask questions, she said. At one checkpoint, police were searching a moving van.

“But I don’t care,” said Lee, 64. “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.”

In Washington, Police Chief Cathy Lanier warned that unattended cars parked in suspicious locations or near critical buildings and structures would be towed.

Secretary of State Hillary Clinton said there was “a specific, credible but unconfirmed report that al-Qaida, again, is seeking to harm Americans and in particular, to target New York and Washington.”

“Making it public as was done yesterday, is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance,” she said Friday morning during a speech at the John Jay College of Criminal Justice in New York.

That the threat is credible but not corroborated means that the information came from a single source, New York Mayor Bloomberg explained Friday during his weekly WOR radio address.

“Corroboration means you get multiple sources, which increases the likelihood that it’s real,” he said. “Credible means that it’s possible to do.”

These sorts of vague descriptions are typical intelligence talk in an environment where tips come from all places and in all shapes— a stolen diplomatic cable, a satellite image showing tribesmen gathering in an area that’s typically isolated, a snatched bit of conversation between two terrorists overheard by a trusted source, a phone number, a document, an email, an airplane ticket.

“Figuring out who would-be attackers are, or even whether they exist, could take months, where the drumbeat of national security wants answers in minutes or days,” said, Phillip Mudd, a former top counterterrorist official at the CIA and the FBI. “”You have to tell everyone what you heard, and then try to prove the information is legitimate.”

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Associated Press writers Christopher Hawley, Colleen Long and Samantha Gross in New York, and Kimberly Dozier, Ben Feller, Jessica Grescko, Matthew Lee, and Eric Tucker in Washington contributed to this report.

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Copyright 2011 The Associated Press.