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. US 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, NY, 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."