(WASHINGTON) -- On the 10th anniversary of the Sept. 11, 2011, terror attacks, the U.S. intelligence and law enforcement apparatus continued a feverish, around-the-clock effort to find evidence to corroborate an unfolding terror plot.
By late Saturday, none was in hand. However, the picture authorities were working from had been fleshed out substantially since the intelligence from Afghanistan arrived Wednesday.
Authorities now believe, like several self-radicalized American jihadists before them, that the Sept. 11 anniversary plotters journey to jihad began in the U.S., took them to the Afghanistan-Pakistan region in July and had them, according to the CIA informant whose information is the hard bottom of the case, return to the U.S. on Aug. 31 or Sept. 1 to carry out an anniversary bomb plot.
So far, no new intelligence has come in to support or discount the informant's account coming in from overseas. The process of elimination has been a grueling one for analysts and law officers working around the clock.
Since last Thursday, authorities have feverishly scrubbed communications and travel manifests using a series of computer search formulas to reduce the pool of possible candidates for investigation from several hundred names to a much narrower group, according to law enforcement and intelligence sources.
Now the FBI Joint Terrorism Task Force (JTTF) has fanned out its resources -- which include those of numerous other agencies -- across the country to find, interview and eliminate people from the pool of potential suspects.
Here is how that process works according to authoritative sources who spoke to ABC News' Pierre Thomas:
The pool of names was culled from hundreds of men who flew into the U.S. from south Asia in August. That list was developed by running names through all terror watchlists and intelligence files available to the U.S. government.
In sifting and narrowing the names, authorities also used a formula that included the ages of potential cell members -- the informant said they would be in their 30s or 40s -- date of travel, and multiple travel routes to and from the Afghan-Pakistan region. From among that group, multiple authorities have told ABC News that two identities are of particular interest and appear possible members of the plot.
But whether the credible information that there is a plot is ultimately proven or disproved depends on whether an informant's information, gleaned secondhand from conversation in a house in Afghanistan, can be verified.
The specific travel time frame information comes from the same CIA informant who said three people were dispatched by al Qaeda to carry out a bomb plot in either New York or Washington.
Despite the secondhand nature of the source, the high quality of the information led authorities to deem the threat credible, if uncorroborated, and triggered the massive rapid police response in Washington and New York, and the all-hands intelligence community and law enforcement manhunt.
"What's striking about this particular information is its clarity," a senior official told ABC News. "Usually intelligence comes in bits and pieces and officials have to connect dots. Here, I'm told, there were no dots to connect, there was so much detail in one place at one time. It was all laid out. If it's a plot it is well-planned and there's an intention to go forward with it. If it turns out not to be real then it's definitely not. There is no middle ground."
As Americans begin to remember the 9/11 anniversary on Sunday, officials say they have no choice but to act as if the threat is real and the clock is ticking.
"Al Qaeda again is seeking to harm Americans and in particular to target New York and Washington," said Secretary of State Hillary Clinton in New York Friday. "We are taking this threat seriously. Federal, state and local authorities are taking all steps to address it."
Police and federal agents have run down stolen vehicles, stopped suspicious trucks, issued "Be On The Look Out's" (BOLOs) for persons suspected of harboring animosity that could be directed at ground zero or the president, on top of continuing the hunt for terror plotters.
The BOLOs were sent for subjects including a 2003 Afghanistan veteran believed armed with handgun and believed to have crossed the George Washington Bridge Saturday morning heading from New Jersey to New York.
That man, missing since Tuesday from his job, had been located in New Jersey by authorities a day earlier, and was the subject of a JTTF BOLO when his EZPass alerted police he had crossed the bridge.
Authorities have also investigated a stolen van on Long Island, a missing rental van near New York, possible stolen vans in Maryland, wiring boxes on bridges, packages and bags left in boroughs outside of Manhattan, airport security breaches at John F. Kennedy International Airport and suspicious packages at Dulles International Airport -- all seen since Friday night and all given extra scrutiny.
In the midst of the scramble Saturday, a Department of Homeland Security analyst sent out a bulletin that multiple sources in law enforcement say erroneously linked a man to the alleged anniversary plot. That document was rapidly pulled back, but the JTTF in North Carolina continued to be besieged by calls.
"We found out it came from an overzealous DHS analyst. Charlotte does have a case on him but he is not one of the three. DHS is going to get spanked. Charlotte is getting inundated with calls," one official told ABC News.
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