WASHINGTON – American fighter jets and Black Hawk helicopters will be circling the capital region Thursday as official Washington gathers for the second inauguration of President Bush – and the first such ceremony since the terrorist attacks of September 11, 2001.

The combat air patrol will be part of an unprecedented security blanket for the 55th inauguration of an American president, including the deployment of Black Hawk helicopters attached to the customs service, patrol boats skippered by the Coast Guard, and F-15s and F-16s from the North American Aerospace Command, or NORAD.

While officials from the Department of Homeland Security, including the outgoing secretary, Tom Ridge, say there are no specific threats to the inaugural activities, the numerous precautions indicate the Secret Service and other agencies are preparing for the worst.

American officials say some 6,000 police officers will be dispatched around Washington as teams of bomb-sniffing dogs and emergency units that specialize in cleaning up biological or chemical attacks set up positions near the parade route.

“With national special security events, we increase security on multiple layers – including air, maritime, and land – with prevention protection and response capabilities,” a Department of Homeland Security official said yesterday.

Major Maria Quon, a public affairs officer for NORAD, yesterday said, “We maintain a robust air-defense posture. We employ a variety of assets and these assets include fighter aircraft, communications systems, radar, and other systems. … Our posture is driven by threat assessment, and a special occasion such as the inaugural is certainly included in this assessment.”

NORAD ran air combat patrols over the Washington, D.C., area after the September 11, 2001, attacks until 2002. Between January 3 and 10, NORAD ran training exercises with F-16 fighter jets flying at low altitudes around Washington and along the Potomac River. Yesterday, Major Quon said two F-16s were dispatched in response to a report of unauthorized aircraft in the region, but they returned to base “after we determined there was not a threat.”

Fox News reported over the weekend that the FBI’s Washington field office had issued an alert to police departments warning of suspicious individuals casing monument areas, taking photographs, and writing descriptive notes.

The Homeland Security official, who declined to be identified, yesterday downplayed the warnings. “We continue to have no information specifically indicating threats around the inauguration. But we will continue to work in partnership with the FBI in order to investigate any suspicious activity,” the official said.

Another Bush administration official with access to intelligence said, “With general casing incidents in Washington, it is hard to prove – as such – that they are casing ahead of an operation, because of the sheer amount of tourists in the D.C. area,” the source said. “It’s almost impossible to prove someone’s bad intentions.

Time reports in this week’s issue that senior officials in the Bush administration received a report that Al Qaeda may plan to use limousines to deliver bombs “equipped with cylinders of flammable gas.” A similar tactic was used with success against the American embassy in Dar es Salaam in 1998.

Mr. Ridge said last week there were no specific threats directed against the inaugural. “There’s very little to talk about,” he said, “because there is no specific threat directed toward the inauguration or the inaugural activities.”

However, Time reported that barriers have been set up to block any explosive-laden vehicles.

Last spring, the FBI and Department of Homeland Security warned that Al Qaeda may try to attack America in anticipation of the elections. Security provided for the conventions last year was considered unprecedented for such a political event.

The author of “Imperial Hubris,” Michael Scheuer, who retired last month from the CIA’s clandestine service, where he served as the chief of the unit devoted to catching Osama bin Laden, said the national security bureaucracy often attached significance to American landmark events, even though Al Qaeda has never used the occasion of an important American anniversary or special event to stage an attack.

“They have never timed any of their attacks to meet our dates,” he said.

However, Mr. Scheuer said the lack of specific threat information should not necessarily be interpreted as the absence of a threat. “Maybe they have improved their communications systems,” he said. “Or maybe they do not need to communicate at this stage of planning for an attack.”

The former CIA official said Mr. bin Laden may have answered some Islamic critics’ concerns about the September 11, 2001, attacks, leaving open the door to a new attack on American soil.

“After September 11, bin Laden was criticized because he did not warn Americans of the attack, give them a chance to convert to Islam, and [because] he killed too many civilians,” Mr. Scheuer said. He said that in his most recent speeches addressed to America, Mr. bin Laden did refer to American policies and addressed Mr. Bush specifically, asking him to lead the country toward Islam.

Also, Sheik Hamid bin Fahd issued a little-noticed fatwa in 2003 that said Al Qaeda could kill up to 10 million Americans because that is how many Muslims have been killed by America and its allies since 1945.