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.

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Counterterrorism officials have been working around the clock to determine whether the threat is accurate, and extra security was put in place to protect the people in the two cities that took the brunt of the jetliner attacks that killed nearly 3,000 people at the World Trade Center and the Pentagon a decade ago. 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 intelligence community can say whether this particular threat is real.

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.

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.

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.

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."

At least one of the three people involved in the plot was thought to be a U.S. citizen, several senior U.S. officials said.