President Barack Obama on Thursday ordered a redoubling of US counter-terrorism efforts in the face of a "credible but unconfirmed" threat ahead of the 10th anniversary of the September 11, 2001 attacks.
US officials, speaking to Reuters on condition of anonymity, said the threat involved Washington D.C. and New York City -- the sites involved in the al Qaeda attacks a decade ago this Sunday that killed nearly 3,000 people.
A law enforcement source said a manhunt was underway for two or three suspects.
But the officials used strong caveats when discussing the threat information privately, with a national security official cautioning that experts thought the threat would ultimately not check out.
New York City Mayor Michael Bloomberg also stressed that the threat had not been corroborated, even as he announced heightened security measures "some of which you may notice, some of which you may not notice."
"There is no reason for any of the rest of us to change anything in our daily routines," he told a news conference.
Still, Bloomberg asked citizens to report suspicious or dangerous activity, adding: "Over the next three days we should all keep our eyes wide open."
The White House said Obama was briefed on specific threat information on Thursday morning, and noted that the government had already "enhanced its security posture" ahead of the anniversary.
"Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information," a White House official said, speaking on condition of anonymity.
White House spokesman Jay Carney said "we're hyper-vigilant to this specific report that's just coming in." He told MSNBC television that the government was taking all necessary precautions, without offering details.
Documents discovered in Osama bin Laden's compound in Abbottabad, Pakistan, after he was killed in a raid in May by Navy SEALs highlighted his persistent interest in attacking the United States around the anniversary of the 2001 attacks. But it is unclear if those plans ever evolved beyond aspiration.
"As we know from the intelligence gathered following the Osama bin Laden raid, al Qaeda has showed an interest in important dates and anniversaries, such as 9/11," said Jan Fedarcyk with the FBI's New York field office.
The Department of Homeland Security, which said only last week that there was no credible information that al Qaeda was plotting an attack around the September 11 anniversary, declined to offer details on the threat.
It cautioned that there were always threat reports before important dates like the anniversary of the September 11 attacks.
"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," spokesman Matt Chandler said.
"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."
A second law-enforcement source played down an ABC News report about missing rental trucks -- saying the vehicles had been recovered and there was no connection to terrorism.