Diane Rokos of Rosslyn still eyeballs inbound jetliners to make sure their landing gear are down. The hijacked airplanes, she seems to recall, were bound for destruction with their wheels up.

Scott Smit of Falls Church still has a two-week stash of food and water in his garage, an escape plan and a family rendezvous point in the Blue Ridge mountains.

Dawn Caskie of McLean still checks out every person boarding the airplane when she flies and keeps a road atlas in her car in case she has to flee an attack.

Four years and multiple catastrophes after Sept. 11, 2001, mental health experts and area residents say that although 9/11 might seem eclipsed by other events and forgotten by the public, its complex of fear remains just beneath the surface -- ready to trigger instantly.

Shoes still must come off in airport security lines, although booties often are available for those who don't want their feet to get dirty.

Metro riders who once abided by an unspoken code to keep their eyes straight ahead now scan one another warily, urged by disembodied announcements to watch, mostly in vain, for sinister activity and packages.

"I hate it," said Sandy Green of the District. "It is very un-Metro. Everybody [usually] sticks to themselves very, very carefully. And we have to be nosy now."

Signs along the interstates command: "Report Suspicious Activity." False alarms proliferate. Fighters are scrambled to pursue stray Cessnas. The Capitol is evacuated. The threat level is adjusted up, then down.

And real disasters, such as Hurricane Katrina, seem to trigger the anxiety anew.

Forty-eight months after terrorists hijacked four airliners and crashed them into the Pentagon, the World Trade Center and a field in rural Pennsylvania, the Washington area remains haunted by calamity, experts say -- reminded of its possibility and still struggling to find context for something that is unresolved.

Four years seems like a fitting span for a human event to run its course, to have a start, a middle and an end. It's the time it takes to get a degree, serve a term in public office. World War I and the Civil War lasted four years. Michelangelo painted the majestic ceiling of the Sistine Chapel in four years.

Indeed, so much else has happened in the past four years. Anthrax. The snipers. The wars in Afghanistan and Iraq. The tsunami. The London and Madrid train bombings. The hurricane.

Memorials to Sept. 11 have been built, pledged and sometimes even vandalized. The Pentagon, where 184 died, was repaired three years ago.

Ground was broken last week on the first reconstruction project at the World Trade Center, where about 2,750 people perished. And a design was unveiled Wednesday to memorialize the plane that was crashed in Pennsylvania, killing 40 passengers and crew members.

In the immediate aftermath of the attacks, many a commentator suggested that the nation had been "changed forever."

Whether the national psyche was permanently altered is a matter for future historians, but experts say there is no question that the imprint of that day remains vivid in the minds of Americans four years later.

"Americans can't ever know that it's going to be over," said Alan Lipman, a clinical psychologist at George Washington University. "And so it creates a kind of continuing low level of threat for which there is no clear answer. And I think we see that bubbling under American society over the last four years."

Arie W. Kruglanski, a professor of psychology at the University of Maryland, said Sept. 11 anxiety might be temporarily "buried under the barrage of events that assert their priority" -- such as the war in Iraq, the price of gas and now the hurricane.

People cannot worry about all these things all the time, he said. "We would go insane."

But the anxiety is there and ready for reactivation by events such as the recent London train bombings.

After that, Kruglanski said, "we thought: 'How about us? Is Washington next? What about the Metro? Are we doing all we can?' "

But some said taking precautions is futile.

"I'm of the mind that if something happens to this city, and it's chemical, it's just, forget about it," D.C. architect Eric Jackman, 49, a Bethesda resident, said outside his downtown office last week.

"We're done for. . . . Whatever's going to hit us is going to be a death blow," Jackman said.

Sandy Green, 52, a Washington interior designer and mother of three, said after seeing off relatives at Reagan National Airport last week: "I'm figuring . . . if there's going to be a nuclear blast somewhere, it's probably going to be Washington, D.C. We're at ground zero. So what's the point?"

But Celeste Myers, 38, an independent technology consultant who lives in Northwest Washington, said she has refused to let anxiety rule her life for the past four years.

"I have just made a decision that I will not live my life in fear," she said last week as she sat on a bench in Lafayette Square, in beautiful September weather, across the street from the White House.

She said she has a strong faith in God.

"That pretty much has guided my lifestyle," she said. "So these external events have not affected what my . . . lifestyle has been."

Brendan Delany and David Seidman, law clerks at Circuit Court in Rockville, sit at Montgomery County's Sept. 11, 2001, memorial.