The operation involved a “big show of force” which included officers armed with heavy weapons patrolling subways and stepped up security and bag checks at major subway and commuter train stations, including Grand Central and Pennsylvania station terminals, the official said. Law enforcement agencies in neighboring states were also involved.

While the operation had been planned some time ago, the forces and tactics deployed were stepped up after local and federal authorities received what they described as “specific and credible” but unconfirmed intelligence about a possible threat to attack the United States by terrorists timed to coincide with the 10th anniversary of the Sept. 11, 2001, Al-Qaeda attacks on New York and Washington.

Meanwhile, US Secretary of State Hillary Clinton on Friday said that Al-Qaeda was behind the threat.

“We are meeting here in New York ... with the news last night of a specific, credible but unconfirmed report that Al-Qaeda again is seeking to harm Americans and in particular to target New York and Washington,” Clinton said in a speech.

In a speech ahead of the anniversary, Clinton said it was impossible to foil every plot and that Al-Qaeda was still capable of regional and international attacks.

However, she said Washington would wage a “relentless” campaign against it and, later this month, would set up Global Counter-Terrorism Forum to gather officials to identify threats, devise solutions and share expertise.

Intelligence about the anniversary threat only surfaced during the course of this week and included possible threats of attacks targeting subways or commuter trains or possible car bomb attacks in New York or Washington, US officials said.

Vice President Joseph Biden said on ABC’s “Good Morning America” program on Friday morning: “We don’t have the smoking gun but we do have talk about using a car bomb.”

Officials have said that US law enforcement agencies are on the lookout for three possible suspects, though it is unclear how much, if any, precise information regarding the suspects’ identities is available to US authorities.

“We have to be concerned. Terrorism is theater and this is a stage, right now probably the world’s biggest stage,” said New York City Police Commissioner Raymond Kelly.

“We have the opening of the 9/11 memorial, the president and two former presidents here, obviously a lot of high profile public officials will be here, so we have to be concerned,” Kelly told CNN.

On Thursday, President Barack Obama ordered a redoubling of US counterterrorism efforts in the face of a “credible but unconfirmed” threat before the Sept. 11 attacks anniversary.

New York City Mayor Michael Bloomberg stressed at a news conference on Friday 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.” Bloomberg asked citizens to go about their daily routines as usual but report any suspicious or dangerous activity.