WASHINGTON - 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.

Secret Service officers speak near the White House in Washington September 9, 2011. [Photo/Agencies]

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.

Security worker Eric Martinez wore a pin depicting the twin towers on his lapel as he headed to work in lower Manhattan on Friday where he also worked 10 years ago when the towers came down. "If you're going to be afraid, you're just going to stay home," he said.

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

Briefed on the threat Friday morning, US President Barack Obama instructed his security team to take "all necessary precautions", the White House said. Obama still plans to travel to New York on Sunday to mark the 10th anniversary with stops that day at the Pentagon and Shanksville, Pa.

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, US officials received information about a threat that included details they considered specific: It involved up to three people, either in the US 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 US 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 US that there was a potential for new terror attacks that could include "places frequented by expatriates and foreign travelers."