NEW YORK (CBSNewYork/AP) -- U.S. intelligence agencies have found no evidence that al-Qaeda has sneaked any terrorists into the country for a strike coinciding with the 10th anniversary of the Sept. 11 attacks, senior officials said.
Authorities still kept a high alert as investigators looked for proof of a plot possibly timed to disrupt events planned Sunday in Washington or New York.
Since late Wednesday, counterterrorism officials have chased a tip that al-Qaida may have sent three men to the U.S. on a mission to detonate a car bomb in either city. At least two of those men could be U.S. citizens, according to the tip.
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.
"We dont know enough to either knock down the alleged threat or confirm it and say it's real so because of that all necesary precautions are being taken," said Bob Orr, CBS News Homeland Security Correspondent.
No intelligence supported that tip as of Saturday, and officials continued to question the validity of the initial information.
1010 WINS' Eileen Lehpamer reports: Police On Heightened Alert
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.
That the threat is credible but not corroborated means that the information came from a single source, 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.''
In the meantime, security is tight in New York City.
From Penn Station, where officers are doing random bag checks and swiping them for gun powder, to the West Side highway, where police are checking nearly every truck that goes by. Officers are also checking vehicles with radiation detectors and license plate scanners.
"We're prepared to be first responders, but we'd much rather be first preventers," said Police Commissioner Ray Kelly.
NYPD officers are working 12 hour shifts for at least the next few days and officials say don't be surprised if you run into check points around the city. Police and military personnel aren't taking any chances.
"We're going to crack down on everything you know that we see" said National Guardsman Chris Cardinali.
One advisory that went out Saturday morning was a report of a Budget rental van stolen from Jersey City. The advisory told all area police departments to be on the lookout for the white 2007 van with the words "Budget Rental" on the side.
The NYPD also says they've received a higher than normal number of calls about suspicious cars and packages around the city and late Friday night, police closed the upper level of the Queensboro Bridge for more than an hour as they checked out some construction equipment.
Long Island police have also heightened security in response to the terror threat.
WCBS 880's Sophia Hall reports: Security Beefed Up On Long Island
New Yorkers say they find the extra security measures comforting, but most say they're just going about their daily lives.
"I'm not overly concerned," said Tuckahoe resident Bob Zamorski. "I think we have threats all the time that we probably don't hear about."
"I'm not going to let no one make me afraid of anything," said one woman. "This is America. We cannot live in fear."
Gov. Andrew Cuomo told CBS 2HD that he moved extra state police to assist the Port Authority and MTA.
"So we are doubly sure that we every kind of protection that is deemed necessary. We're doing that," he said. "We're very good at doing this. Don't let it impose or infringe on tomorrow."
Cuomo added that despite the extra security, New Yorkers shouldn't forget the importance of this weekend.
"We shouldn't allow this threat to diminish the import of the 9/11 anniversary because that would be doing just what the terrorists want us to do," he said.
Officials say al Qaida is weaker since the death of Osama Bin Laden, and that U.S. anti-terrorism efforts have grown stronger.
"There have been been dozens of plots attacks and networks disrupted since 9/11 things that we will not know about," said Juan Zarate, CBS News Security Analyst.
What's your take on all the heightened security? Sound off in our comments section below…
(TM and Copyright 2011 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2011 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)
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