Their primary mission is to explode a car bomb in either New York or Washington, but if that proves impossible, they have been ordered to simply cause as much destruction as they can, one U.S. official said.
Word that al-Qaida had dispatched would-be attackers reached U.S. officials midweek. A CIA informant who has proven reliable in the past approached intelligence officials overseas to say that the men had been ordered by newly minted al-Qaida leader Ayman al Zawahri to mark the 10th anniversary of the 9/11 attacks Sunday by doing harm on U.S. soil.
The tipster says the would-be attackers are of Arab descent and may speak Arabic as well as English. Counterterrorism officials were looking for certain names associated with the threat, but it was unclear whether the names were real or fake.
Counterterrorism officials have been working around the clock to determine whether the threat is accurate, but so far, have been unable to corroborate it, the officials said, speaking on condition of anonymity to discuss the investigation.
In the meantime, 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.
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.
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, 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.
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.
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."
Associated Press writers Lolita C. Baldor, Christopher Hawley, Colleen Long and Samantha Gross in New York, Ben Feller, Jessica Grescko, Matthew Lee, and Eric Tucker in Washington contributed to this report.