WASHINGTON -- American spy networks have intercepted communications from an al Qaeda operative in Pakistan indicating plans for a terrorist strike in New York or Washington, according to a senior U.S. official.
The communications come from a source who has provided accurate information in the past, the official said, prompting intelligence officials to sift through communications from al Qaeda cells.
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No other corroborating evidence of an attack has been uncovered, the official added.
But the information indicates the plan -- thought to involve a vehicle-borne explosive device -- is meant to coincide with the 10th anniversary of the September 11 attacks.
Terror threat on 9/11 real? Still, intelligence officials said they have picked up "chatter," or widely divergent communications, from extremists that suggest the newly tapped al Qaeda head, Ayman al-Zawahiri, is involved in the current plot.
Meanwhile, heightened security was visible Friday in New York and Washington as intelligence officials worked to pin down information about the potential threat and federal and local law enforcement fanned out across both cities. Police checkpoints jammed Manhattan streets, causing traffic delays.
A senior U.S. official said the plot was believed to involve three individuals, including a U.S. citizen. It is believed to entail a vehicle bomb, but "we cannot rule out other means."
U.S. Vice President Joe Biden also weighed in Friday, saying the information was at least in part gleaned from the May raid against al Qaeda leader Osama bin Laden.
"This is the first ... credible piece of information we've gotten," Biden told NBC's "Today" show. "All hands are on deck."
It is not clear how the bin Laden raid helped authorities connect the dots to the prospect of an anniversary attack, but Biden downplayed the threat of a widely sophisticated plot involving multiple actors.
He said the administration's principal concern from a "lone actor, not some extremely complicated plan like it took to take down the World Trade (Center) towers or the plane in Shanksville (Pennsylvania) or the Pentagon."
He added, "It doesn't mean they couldn't happen, but it's much less likely. The lone actor is the more worrisome thing because there are fewer trails to follow, there are fewer leads to move on."
On Friday morning, police officers stopped and searched box trucks as they approached the George Washington Bridge, which links New Jersey with New York, as well as by the Queens Midtown Tunnel, the Holland Tunnel and the Verrazano-Narrows Bridge, connecting the boroughs of Staten Island and Brooklyn.
Police also manned checkpoints in Midtown and downtown Manhattan, where they examined vehicles, particularly vans and trucks, for possible bombs.
"That's something we've always been concerned about, the introduction of a radioactive isotope, a dirty bomb," said New York Police Department spokesman Paul Browne.
New York police also are sweeping parking garages for explosives and using digital license plate readers to check for stolen vehicles, Browne said. While security would normally have been raised for the anniversary of 9/11, he added, the vehicle checkpoints were a response to the threat alert.
A senior U.S. administration official said Friday that it is the "origin" of the intelligence information that is causing particular concern.
The "working assumption" is that if the plot is genuine, then some individuals involved in it already have entered the United States, the official said. But he stressed they cannot confirm that has happened.
U.S. officials believe operatives came out of the tribal Pakistan-Afghanistan border region -- a volatile semilawless area home to extremist groups -- and that they are part of al Qaeda "central," he said.
The official also noted a possibility that Pakistan-based groups such as Lashkar e-Tayyiba or Tehrik-i-Taliban Pakistan could be involved.
And yet a separate senior law enforcement official involved in briefings about the matter told CNN his best information is that the three individuals had not yet entered the United States.
A senior White House official said President Barack Obama had been updated about the potential threat after his speech Thursday night to a joint session of Congress, and White House counterterrorism adviser John Brennan would brief him again Friday.
No changes have been made to the president's schedule for Friday or the weekend, the official said.
Asadullah Khalid, Afghanistan's minister of borders and tribal affairs, said no recent intelligence suggests a possible attack on the United States.
New York City Michael Bloomberg told reporters Thursday night that while additional police will be deployed around the city, "There's no reason for any of the rest of us to change ... our daily routines."
New York Gov. Andrew Cuomo urged residents to be cautious but said, "There is no reason to panic."
Another senior federal law enforcement official involved in counterterrorism efforts told CNN that authorities "aren't sure if this is or real or just chatter."
A joint intelligence bulletin issued by the FBI and the Department of Homeland Security indicated other methods than a truck or car bomb could be used, including attacks with small arms, homemade explosive devices and poisons.
Al Qaeda probably would provide its operatives with enough autonomy to select the particular target and method of attack, the bulletin said, and an attack may involve operatives carrying U.S. documentation.
Intelligence officials believe al Qaeda "likely maintained an interest since at least February 2010 in conducting large attacks in the Homeland timed to coincide with symbolic dates, to include the 10-year anniversary of the 9/11 terrorist attacks," the document said.
Bin Laden's death and the "removal" of senior al Qaeda figures since then could add to the organization's desire to stage an attack on a symbolic date such as September 11, the bulletin said.
In Washington, local officials said the city has a robust plan in place and has been on heightened alert since September 1.
"I want to urge all of our citizens ... to remain calm and let our law enforcement professionals do their job. At the same time, we ask people to remain vigilant," Mayor Vincent Gray said. "If you see something, say something."
Unattended vehicles around key infrastructure will be towed, Washington Metropolitan Police Chief Cathy Lanier said.
Federal officials said they were taking the threat seriously, while trying to temper the news by saying such threats are commonplace in connection with key dates.
Rep. Peter King, R-New York, chairman of the House Homeland Security Committee, said specific details were revealed to lawmakers about the threat. "Many agencies are looking at this from every possible angle," he said.
But it is not known yet if the threat is real, he said, adding, "I would tell people now to go about their lives. There's no need to panic."
Homeland Security Secretary Janet Napolitano told reporters Thursday that intelligence officials had picked up "lots of chatter" on jihadi websites and elsewhere about the impending 10th anniversary of the September 11 attacks "and we're taking it all seriously," but nothing yet that warranted issuing a threat advisory.
Nonetheless, she said, the department will be at a heightened level of readiness as the nation commemorates the anniversary, "staffing up" the Federal Air Marshal Service and other agencies.