WASHINGTON — Al Qaeda may have sent American terrorists or men carrying U.S. travel documents to launch an attack on Washington or New York to coincide with memorials marking the 10th anniversary of Sept. 11, government officials say.

One U.S. official says al Qaeda dispatched three men, at least two of whom could be U.S. citizens, to detonate a car bomb in one of the cities. Should that mission prove impossible, the attackers have been told to simply cause as much destruction as they can. But U.S. intelligence officials say they have no evidence there is anyone inside the United States tied to the plot.

Although the initial tip suggested terrorists, including U.S. citizens, may be travelling to the country, that remains unconfirmed.

Word that al Qaeda had ordered the mission reached U.S. officials midweek. A CIA informant who has proved reliable in the past approached intelligence officials overseas to say that the men had been ordered by newly minted al Qaeda leader Ayman al-Zawahri to mark the 10th anniversary of the 9-11 terrorist 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.

Intelligence analysts have looked at travel patterns and behaviours of people entering the country recently. And while they have singled out a few people for additional scrutiny, none has shown any involvement in a plot.

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 U.S. history, and al Qaeda 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.

"We're watching," James McJunkin, FBI assistant director in charge of the Washington field office, said Saturday. "We expect we're going to get an increase in threats and investigative activity around high-profile dates and events. He added: "This is a routine response for us. It's routine because it's muscle memory."

For months, the FBI had planned to increase staffing around the anniversary and police knew they were going to be out in force in Washington, he said.

In New York on Friday, security worker Eric Martinez wore a pin depicting the twin towers on his lapel as he headed to work in lower Manhattan 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 planned to travel to New York on Sunday to mark the 10th anniversary with stops that day at the Pentagon and Shanksville, Pennsylvania.

Washington commuters were well aware of the terror talk.

Cheryl Francis, of Chantilly, Virginia, 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 Qaeda, 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 travelling to the U.S. that there was a potential for new terror attacks that could include "places frequented by expatriates and foreign travellers."

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.

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