NEW YORK — After intensive checking of records and sources, there is still nothing to confirm a Pakistani intelligence source's warning of a vehicle bomb plot around the 10th anniversary of the Sept. 11 terrorist attacks, several intelligence, law enforcement, and Homeland Security officials said Saturday night.
Officials are following up on a tip that came in late Wednesday that al-Qaida may have sent three men to the United States on a mission to detonate a car bomb to disrupt events in New York City or Washington D.C. At least two of those men could be U.S. citizens, according to the intelligence, which security officials emphasized was unconfirmed.
Airline passenger arrival records are being checked against the few vague details the source offered, including age range, approximate heights, fragments of names and travel patterns. That chore is complicated by the possibility that the men may have been smuggled into and out of Pakistan, leaving no record they were ever there.
Law enforcement officials said hundreds of recent arrivals have already been checked and ruled out. Some of that can be done merely by reference to the records, but in other cases, in-person interviews are being conducted.
Interviews by FBI joint terrorism task forces across the country are for elimination purposes, a government official said.
"We do not yet know if any terror suspect entered the U.S. at all," he said. Initial intelligence suggested some suspects were U.S. citizens, the official said, noting the theory is they are traveling on U.S. passports.
It will take a lot longer to know whether the plot was for real or not.
"It's tough to downgrade things at this point. But at the same time, there's nothing to upgrade it either," one official said.
The terrorism tip reportedly came from a CIA informant who has proved reliable in the past, according to U.S. officials. They said the informant approached intelligence officials overseas to say that the men were ordered by new al-Qaida leader Ayman al-Zawahri to mark the 10th anniversary of the Sept. 11 by doing harm on U.S. soil.
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Al-Zawahri took over as the group's leader after the U.S. killed Osama bin Laden during a raid in May at his compound in Pakistan.
On Saturday around New York and Washington, officials said, the FBI has found no sign of unusual purchases of chemicals needed to make car bombs.
Saturday also brought several reports of police seeking stolen vehicles in the New York and Washington areas.Earlier on Open Channel: Two terror threat subjects could be American
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It was already part of the security plan around the 9/11 anniversary to pay extra attention to stolen vehicles, and the intelligence warning about a possible car bomb plot added an extra level of interest.
Washington, D.C., police said on Saturday that in response to all the "see something, say something" admonitions, reports of suspicious activity and vehicles around the District were up 60 percent.Story: Terror threat called 'credible' for 9/11 anniversary
The fact that police are looking for any particular stolen vehicle doesn't mean much, because they're following up on all of them.
In New York, police were seeking two missing dark green Chevrolet vans with red stripes used in city roadwork, but there was no direct connection to the terror threat at this time, a New York law-enforcement official said. Instead, authorities suspect an inside job. One van taken from the Long Island Expressway at the Utopia Parkway exit on Sept. 1 or Sept. 2 apparently was later used to steal $70,000 in construction equipment.
In New Jersey, a law enforcement official said, "We may have a stolen vehicle but nothing to indicate it has anything to do with terrorism." Reports were being checked out as a precaution, he said.
Jonathan Dienst is WNBC's chief investigative reporter. Pete Williams is NBC News’ senior justice correspondent.