The threat information was obtained recently and originated from overseas sources -- Pakistan, according to one official.
According to a White House official, "The President was briefed on this specific threat information this morning and has been updated on it throughout the day. The United States government has already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats. Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."
Earlier this evening, top Congressional leaders were briefed on the terror threat by White House counterterrorism advisor John Brennan.
In a statement, Department of Homeland Security spokesman Matthew Chandler said, "As we know from the intelligence gathered from the Osama bin Laden raid, al Qaeda has shown an interest in important dates and anniversaries, such as 9/11. In this instance, it's accurate that there is specific, credible but unconfirmed threat information."
"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days," said Chandler. "Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots underway. Regardless, we take all threat reporting seriously, and we have taken, and will continue to take, all steps necessary to mitigate any threats that arise."
U.S officials have been bracing for just such a plot after discovering documents in bin Laden's Pakistan hideout in May in which the al Qaeda leader called for a strike against the U.S. on the tenth anniversary of September 11th.
In Zawahiri's most recent video, last month, he called for his followers to focus on the U.S.
"We know from Bin Laden's own handwriting he wanted to do an attack around the anniversary," said Richard Clarke, former White House counterterrorism advisor and now an ABC News consultant. "We know from his successor's own audio tapes and video tapes that he feels he has to prove al Qaeda is still alive by avenging bin Laden's death. And we know that this kind of technique could be relatively easily done, even by an al Qaeda that's on the ropes, even by an al Qaeda that has very few people left."