Officials say at least three individuals are being sought. One of the suspects is believed to be an American citizen. They are suspected of entering the country in August.
Intelligence officials believe the suspects entered the country with the intent to launch a vehicle-borne attack around the 10-year anniversary of the attacks of September 11, 2001.
The information on the plot was "very specific," said 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."
One of the intelligence and law enforcement officials that ABC News spoke to said that one of the individuals may be a U.S. citizen and a second official said that possibly two of the individuals may have had U.S. documentation -- whether green cards or passports was unclear.
Authorities were searching for at least two rental trucks Thursday evening, a Penske and a Budget, in the country, but did not specify where or if that search was connected to the plot. A short time later both trucks were located. They appeared not to be connected to those being sought.
As news of the information became public, authorities began weighing whether to raise the national threat level.
The suspected terrorists are believed to have embarked on their journey to the U.S. from Afghanistan or Pakistan, according to two senior officials. One official said the route may have taken them through at least one other country, possibly Iran. Another news organization reported that they embarked from "the Pakistan area."
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 under way. 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."
Security visibly increased around the Washington D.C. on Thursday and police intend to increase their staffing.
The FBI and Department of Homeland Security sent bulletins to 18,000 law enforcement agencies warning them of a potential terrorist plot.
New York City Mayor Michael Bloomberg held a news conference Thursday night. He stated the threat was "credible, but not corroborated."
"Over the next few days we should all keep our eyes wide open," said Bloomberg. But, he added "For 10 years we have not allowed terrorists to intimidate us."
New York City Police Commissioner Ray Kelly also spoke at the news conference. He said there would be increased traffic checkpoints as a result of the possible threat and as the 9/11 anniversary approaches.
The police response, Kelly said, would also include increased radiation and bomb detection, and increased monitoring of tunnels, bridges, infrastructure, city landmarks and government buildings.
There will also be increased towing of illegally parked cars, Kelly said.
