Feds: Zawahiri Launched Al Qaeda Terror Plot for 9/11 Anniversary Mayor Bloomberg: "We have to not let the terrorists win by intimidation."

Sept. 8, 2011  -- U.S. authorities are scrambling to sort through information that the CIA developed in the past 24 hours indicating that at least three individuals entered the U.S. in August by air with the intent to launch a vehicle-borne attack against Washington, D.C. or New York around the anniversary of 9/11, according to intelligence officials.

Officials say the alleged terror plot was initiated by new al Qaeda chief Ayman al-Zawahiri, Osama bin Laden's successor, who had pledged to avenge bin Laden's death earlier this year in a U.S. raid.

Intelligence and law enforcement officials told ABC News that at least one of the individuals is a U.S. citizen and one official said that two of the individuals may have had U.S. documentation -- whether green cards or passports was unclear.

The threat was described as specific and credible by federal officials. "Al Qaeda has shown an interest in important dates and anniversaries. In this instance it is accurate that there is credible, specific but unconfirmed information," said Janice Fedarcyk, FBI Assistant Special Agent in Charge of the New York office, at a Thursday evening press conference.

At a the same press conference, New York City Mayor Mike Bloomberg said the NYPD was already on alert because the anniversary is seen by terrorists as an opportune time to strike, and would be deploying additional resources around the city. He also said that the threat was credible and had to be taken seriously, though "it has not been corroborated."

"We do live in a world where we must take these threats seriously and we certainly will," said Bloomberg.

As news of the information became public, authorities began weighing whether to raise the national threat level. The FBI and the Department of Homeland Security have sent a bulletin to 18,000 law enforcement agencies around the country warning them of the potential plot. The bulletin notes that al Qaeda might seek revenge for the death of bin Laden and other key figures on a symbolic date like the anniversary of 9/11.

Although authorities said they had not yet identified the suspects, they are looking at multiple names of individuals who entered the U.S. after mid-August. The operatives are believed to have embarked on their journey to the U.S. from the tribal areas of Pakistan, according to two senior officials. One official said the route may have taken them through Dubai.

The information on the plot was "very specific " said one official, adding " It seems like the information has been worked for a while." A second and third official confirmed the specifics of the possible plot added that a declassified bulletin is expected to be issued shortly.

Initially, one official added that at least two rental trucks -- one from Penske and one from Budget -- were being sought nationwide. Those trucks have since been recovered and appear to have had no connection to the plotters.

Obama, Congress Briefed On Threat

The threat information was obtained recently and originated from overseas sources -- Pakistan, according to one official.

According to a White House official, "The President was briefed on this specific threat information this morning and has been updated on it throughout the day. The United States government has already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats. Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."

Earlier this evening, top Congressional leaders were briefed on the terror threat by White House counterterrorism advisor John Brennan.

In a statement, Department of Homeland Security spokesman Matthew Chandler said, "As we know from the intelligence gathered from the Osama bin Laden raid, al Qaeda has shown an interest in important dates and anniversaries, such as 9/11. In this instance, it's accurate that there is specific, credible but unconfirmed threat information."

"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days," said Chandler. "Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots underway. Regardless, we take all threat reporting seriously, and we have taken, and will continue to take, all steps necessary to mitigate any threats that arise."

U.S officials have been bracing for just such a plot after discovering documents in bin Laden's Pakistan hideout in May in which the al Qaeda leader called for a strike against the U.S. on the tenth anniversary of September 11th.

In Zawahiri's most recent video, last month, he called for his followers to focus on the U.S.

Click Here to Sign Up for Breaking News and Investigation Alerts From The Brian Ross Investigative Unit

"We know from Bin Laden's own handwriting he wanted to do an attack around the anniversary," said Richard Clarke, former White House counterterrorism advisor and now an ABC News consultant. "We know from his successor's own audio tapes and video tapes that he feels he has to prove al Qaeda is still alive by avenging bin Laden's death. And we know that this kind of technique could be relatively easily done, even by an al Qaeda that's on the ropes, even by an al Qaeda that has very few people left."

'We Have to Not Let the Terrorists Win'

In New York, police commissioner Ray Kelly outlined a series of stepped-up security measures the NYPD would undertake, even beyond those already in progress in preparation for the tenth anniversary of Sept 11th.

"We will be holding personnel, holding our tours," said Kelly. By adding four hours to those tours he effectively increased the amount of cops on duty by a third.

He indicated increased vehicle checkpoints would be in effect, and that more bomb dogs and radiation detectors would be in use. "The public is likely to see and maybe somewhat inconvenienced by vehicle check points at various locations throughout the city," said Kelly.

There will be also be stepped up bag searches, towing of illegally parked cars and increased police presence on the ferries, Kelly said.

"Over the next few days we should all keep our eyes wide open," said Mayor Bloomberg.

But, he added, "For ten years we have not allowed terrorists to intimidate us. ... There is no reason for any of the rest of us to change our daily routines. Over the past ten years, [the NYPD] has helped thwart over a dozen potential attacks. The best thing we can do to fight terrorists is to refuse to be intimidated by it."

"We have to not let the terrorists win by intimidation," said Bloomberg.

Aaron Katersky contributed to this report.

Click Here for the Blotter Homepage.