President Obama was briefed on the threat on Thursday morning, the White House said. Two U.S. officials briefed on the intelligence say a tip was received from an area rich with terrorist training camps in Pakistan that several men had entered the United States within the past few weeks with the intent of detonating explosives in cars in Washington and New York City.
The officials said that there was no corroborating information and would not say whether the threat was picked up by a U.S. source or was provided by a foreign government.
"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," a White House official wrote in an e-mail. "Nevertheless, the president directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."
Police in both cities were briefed this morning, but the two departments already were on heightened alert because of the impending anniversary.
"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days," Matt Chandler, a Homeland Security Department spokesperson, said in a statement.
"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."