The head of the FBI said Tuesday his agency has "been able to eliminate some aspects" of the threat to the U.S. homeland that emerged in the days before the tenth anniversary of the Sept. 11, 2001, attacks, but he and other counterterrorism officials said they are not yet ready to dismiss the "outstanding" threat.
"Since we first had word of that threat, we have conducted hundreds of interviews, we have been pursuing a number of leads, and consequently -- as a result of that -- we've been able to eliminate some aspects where we thought that we ought to be looking in order to be able to determine if it was indeed a valid threat," FBI Director Robert Mueller told the Senate Homeland Security Committee. "But there's still work to be done."
Intelligence obtained last week suggested Al Qaeda may have sent as many as three operatives to the United States to detonate a car bomb in New York City or Washington, but as of Tuesday afternoon -- several days after first learning of the possible plot -- authorities had yet to substantiate any of the information received.
Still, the new head of the National Counterterrorism Center, Matthew Olsen, echoed Mueller's remarks, saying U.S. officials are "not prepared" to say the threat "has been resolved" as they "continue to work to analyze it and share information about it."
This comes three days after a senior U.S. official familiar with the intelligence told Fox News the threat "is looking less and less credible."
In particular, the official said some specifics of the alleged plot don't seem "feasible," and that trying to confirm the threat is "looking more and more like a goose chase."
On Tuesday, Mueller emphasized the FBI, Department of Homeland Security, NCTC and other intelligence agencies are still "pursuing" the threat "as heavily as we have in the last several days, and we will continue to do so until it is resolved."
He said even though the Sept. 11 anniversary passed without major incident, officials "do not believe that necessarily means that we should back down." DHS Secretary Janet Napolitano said authorities "continue to lean forward into confirming that threat."
Senate Homeland Security Committee chairman Joe Lieberman, I-Conn., asked Mueller specifically whether -- with such a specific and credible intelligence stream -- the FBI and others were prepared to dismiss it. Mueller answered simply, "No."
The timing of this particular threat had officials especially concerned, as it was the first credible threat information to emerge in the run-up to the significant anniversary.
The U.S. government has long known that terrorists see the anniversary of Sept. 11 and other uniquely American dates as opportunities to strike, and information gleaned in May from Usama bin Laden's compound in Pakistan further confirmed that Al Qaeda had been eyeing the tenth anniversary. Officials were also concerned that some may see this anniversary as an opportunity to avenge bin Laden's death.
-- Fox News' Jennifer Griffin and Catherine Herridge, and FoxNews.com staff, contributed to this report