To fortify New York and Washington on Sunday, federal and local law enforcement officials are piling security plans atop security plans, making it not just the 10th anniversary of the Sept. 11 attacks, but also a milestone in a decade-long state of alert.

The defense portfolio includes thousands of New York and District of Columbia police officers, including divers, bomb technicians and counter-snipers; National Guardsmen; and F.B.I. intelligence analysts. The commemoration Sunday at the site where the World Trade Center was destroyed, and where a new memorial will be unveiled, will be attended by President Obama, former President George W. Bush and other dignitaries. President Obama will visit the Pentagon’s 9/11 memorial later Sunday.

Plans for the show of force stretch back at least to May, when a notebook filled with Osama bin Laden’s musings about a possible terror strike on the anniversary of the attacks was discovered in his compound.

But they intensified last week upon word of a new threat. Intelligence analysts on Saturday were poring over aviation and other travel records in an attempt to identify two men, both American citizens, whom a Central Intelligence Agency informant heard had been dispatched by Al Qaeda to mount car bomb attacks in New York or Washington, officials said.

In the jargon of threat assessment, the report was called “credible” because the source has been reliable in the past, and “specific” because it described a mode of attack and geographic targets. But it was also labeled “unconfirmed” because it came from one source whose information was second- or third-hand.

On Saturday afternoon, federal officials said they had found no evidence that a Qaeda operative had entered the United States.

Still, the security measures were unmistakable: checkpoints at bridges and tunnels; police and Coast Guard boats around Manhattan; and heavily armed officers at transportation hubs like Grand Central Terminal and Pennsylvania Station.

Overhead, the whir of police helicopter rotors was heard. And military combat aircraft, under orders given well before the newest threat emerged, began their patrols in the skies above New York and Washington.

In Lower Manhattan, an officer working with others at a checkpoint — asking random drivers to step out of their vehicles, open their trunks and describe the contents — said the security mission had, in a way, become almost routine, given previous threats over the past 10 years. “We’ve done this before,” the officer said, shrugging.

Near City Hall, metal barricades lined sidewalks, and a roadside message board warned approaching vehicles with flashing orange letters: “Avoid Downtown.” A team of bomb squad officers with a German shepherd and two Labrador retrievers patrolled the north end of the World Trade Center memorial site.

In Washington, too, the police threw a formidable blanket over the city, towing unattended cars and trucks, sending bomb-sniffing dogs into the subway and deploying several hundred extra officers on the streets. Cathy Lanier, the police chief, said there had been a surge of calls to report suspicious vehicles and behavior, and police were checking every tip.

As Sunday drew closer, local and federal officials stepped up their less-visible safeguards, too. Agents and police detectives from the F.B.I. Joint Terrorism Task Force were working around the clock to run down leads; police officers from the Intelligence Division were visiting suppliers of goods and services that may be sought by terrorists, like ammonium nitrate for a bomb and rental agencies to deliver it.

At New York police headquarters, a new Joint Operations Center was activated, so officials from 30 agencies, including the Secret Service, could work face to face with police commanders and officials from other city agencies.

In addition, the police were giving extra scrutiny to reports of stolen trucks and vans, said Paul J. Browne, the department’s chief spokesman. Specifically, investigators were working to find a Budget rental van, with Oklahoma license plates, that was taken from a lot in Jersey City on Aug. 21 by thieves who cut lines to the phone and alarm systems and tampered with the security cameras. They were also trying to find two dark-colored vans that were loaded with expensive tools and stolen this month from a construction company with a contract to do road work on the West Side Highway, near ground zero.

“These may be nothing more than industry-savvy thieves with an appetite for expensive construction tools,” Mr. Browne said. “But they’re receiving greater scrutiny in order to eliminate the possibility of something more sinister.”

As the hours ticked by, top officials huddled to assess the latest information and gauge the efforts to prevent any attack. On Friday, Police Commissioner Raymond W. Kelly took progress reports around a visit to a mosque in upper Manhattan, where he spoke of interfaith understanding. On Saturday, he briefed the Homeland Security secretary, Janet Napolitano, at police headquarters, and Mr. Obama, in Washington, met with his national security team, the White House said.

Stewart A. Baker, a top Homeland Security official from 2005 to 2009, said that despite the frustrating nature of the vague threat report, panic had not seemed to set in. “No one’s staying home in dread,” he said. ”I think there’s sort of a mental toughness now.”

(Source: NY Times)