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 9/11, 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 traveling 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 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 behaviors 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.

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