The threat -- described as “specific" and "credible" -- has not yet been confirmed, reports NBC.
ABC News has more details. Intelligence officials told ABC that three individuals entered the country with the intent of launching a 'vehicle-borne attack' against New York or Washington D.C., around the anniversary of 9/11.
According to sources who spoke with ABC, the suspected terrorists may have originated from Afghanistan.
Mayor Michael Bloomberg will hold a press conference with the NYPD and FBI at 9:30 p.m. Thursday evening.
CNN reports that the FBI and the DHS are trying to put out a bulletin to law enforcement officials overnight regarding a possible attack.
More from HuffPost reporter Andrea Stone:
Matthew Chandler, spokesman for the Department of Homeland Security, confirmed to The Huffington Post that there is a "specific, credible but unconfirmed threat" of an attack on 9/11.
Based on intelligence gathered from the Osama bin Laden raid in May, al Qaeda "has shown an interest in important dates and anniversaries, such as 9/11," he said. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days. 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. We continue to ask the American people to remain vigilant as we head into the weekend.”
A US intelligence official told HuffPost, "US counterterrorism entities have been alert for any leads, and for the first time we’ve received specific and credible—but unconfirmed—threat information linked to the 9/11 anniversary. Any and all leads related to possible plots are of course being run to ground, and the US will bring every resource to bear to ensure the security of the American people.”