At 200 feet, the U.S. Park Police helicopter skims over the Tidal Basin, low enough to study the tourists standing on the steps of the Jefferson Memorial and the vehicles on nearby streets. When the crew members look down nowadays, they view the nation's most cherished monuments in a disturbing light -- as possible terrorist targets.

"We're looking for the unusual," said Sgt. Kenneth Burchell, the pilot. "What looks wrong?"

Increased patrols are only part of the picture. In the past 18 months, the region's top officials have spent countless hours and hundreds of millions of dollars on a massive undertaking that has few guidelines and no real completion point.

Their goal is not only to prevent another terrorist assault on the nation's capital, but also to plan for any possible aftereffects.

With the threat that terrorists could retaliate during a U.S. war against Iraq, some believe this planning soon could be put to the test.

Officials agree that Washington is much safer and better prepared for such a strike than it was Sept. 11, 2001. Throughout the region, police departments, hospitals and government agencies have been working to strengthen security measures, improve communication lines and stockpile medicines and protective gear. But they have also been forced to come to terms with an enemy who defies planning or prediction.

"This is a new area -- it's not like designing the next fighter plane where there's a previous long history of fighter planes," said Ralph Gomory, president of the Alfred P. Sloan Foundation, a nonprofit institution with a long interest in biodefense. "There's very little history and therefore people are doing a complicated thing for the first time, and that's really hard."

A Starting Point

Much of the focus has been on preparing for attacks involving weapons of mass destruction: the explosion of a so-called dirty bomb, the release of a deadly chemical or the spreading of a biological agent such as smallpox. Even though such incidents may not be the most likely, they have received the most attention because they carry the gravest consequences.

The region's leaders also have worked to harden the most obvious targets -- bolstering defenses at the U.S. Capitol, the national monuments and the Metrorail system, among other places -- while conceding that they cannot possibly protect everything.

Officials have locked down manholes, secured underground tunnels and installed sensors along area rivers and roads that can detect radioactive materials. But even as they deploy the latest technological devices and strategies, they also face a stark reality about their mission. In many cases, without specific intelligence beforehand, there may be little they can do to stop a terrorist act from occurring. The preparations and enhanced security serve mostly to deter would-be attackers and, failing that, to detect, limit and recover from any attack as quickly as possible.

Citing security reasons, officials will not divulge many details about beefed-up patrols, newly installed cameras and other measures. But even a partial listing of the changes is eye-opening.

Metro has begun installing chemical sensors that will eventually guard every station. Hood respirators have been purchased for every police officer, congressional staff member and tourist at the Capitol. Each of Inova's four hospitals has enough decontamination showers to treat 500 patients an hour.

To monitor traffic and suspicious activity at key intersections, the District's transportation agency has started buying 100 remote-control digital cameras. Washington Hospital Center is building quarantine facilities. A grass-roots movement is underway to establish a network of two-way radios that would relay neighborhood information should cell phones and e-mail become inoperable.

Training exercises in this tense climate have become elaborate affairs: In January, on the weekend before the president's State of the Union message, 700 police and emergency workers staged an eight-hour overnight exercise inside the Capitol that simulated what would happen if a nerve gas were released during the speech.

Some officials see a psychological as well as practical value in all the drilling and planning.

"This is not [only] about our ability to respond or recover. It's about our ability to manage fear," said George W. Foresman, deputy assistant for commonwealth preparedness for Virginia Gov. Mark R. Warner (D). "Al Qaeda has been quite successful in using Americans' ability to imagine and to operate with limited information to create fear. Our key tool to take that weapon away from the terrorists is to enhance our preparedness."

It is a complicated task to coordinate preparedness for such an unusual area. The capital region comprises three federal branches of government, two states, a federal district, 17 local jurisdictions, hundreds of federal and local agencies, 350,000 federal workers, 60 colleges with a quarter-million students and 20 separate police and federal protective service forces. With the Pentagon attack, flaws in communication lines and other areas were quickly exposed, and officials worked to clear up the obvious weaknesses.

"If you're talking about preparedness in the Washington area as a whole, one of the critical things you've got to focus on is how coordinated are these players?" said Lara Shane, a spokeswoman for the Federal Emergency Management Agency. "That's where a lot of work has been done."

Looking for Answers

But state and local officials say they are still bedeviled by a shortage of intelligence about what terrorists are planning and when and where it might occur. Lacking specific threat information, they have made it part of their job to imagine the worst.

"We sit here and we try to think like a terrorist, and that's a difficult leap for us," said Mark Marcotte, who as deputy general manager and chief engineer of the D.C. Water and Sewer Authority is responsible for the safety of the 150 million gallons of water District residents use each day.

To that end, regional officials have studied horrific scenarios on an almost weekly basis. They have explored what would happen if a school bus bearing a dirty bomb were to detonate outside the National Air and Space Museum; if anthrax spores were released in the Metro; if the National Institutes of Health's main laboratory building blew up; if the Woodrow Wilson and American Legion bridges were attacked simultaneously by truck bombs. They have tried to figure out how they would keep transportation routes open, deploy police officers and store the mountains of debris during a cleanup.

The Senate has even conducted a mock session at an alternate chamber inside the District, a facility equipped with television feeds so that its leaders could quickly reassure constituents across the country.

"Anything you could reasonably expect terrorists would throw at us, this area is prepared to handle," U.S. Capitol Police Chief Terrance W. Gainer said.

The sewer authority assessed whether someone might try to put a chemical or biological agent into the water supply. In fact, that is difficult to accomplish, because the levels needed to contaminate such large volumes of water would require attention-getting tanker trucks instead of the small eyedroppers the public might envision, Marcotte said.

The authority recently spent $5 million on sensors, cameras and perimeter security, but one of the biggest changes took place the day after the Pentagon attack. At the Blue Plains wastewater treatment plant, officials decided to replace the 90-ton tanks of chlorine gas -- which, if exploded, could send toxic fumes over a wide area -- with a relatively harmless liquid bleach product.

"Without taking anything for granted, the water system in general is, we believe, a relatively secure system," Marcotte said. "That's not to say we feel immune, because we don't."

Special training has become paramount. Capitol Police, for example, are setting up a 60-member hazardous material unit, the largest such law enforcement unit in the country. Another 160-officer team was trained for rescue and decontamination work.

Police are ready to deploy an array of sophisticated gear, ranging from stationary monitors that sniff the air for toxins to portable mass spectrometers that can identify thousands of molecular compounds within moments to hand-held devices that record a person's radiation exposure.

Just last weekend, they got a chance to test their training. A suspicious, 55-gallon drum was left at Vermont and H streets NW, two blocks from the White House. Secret Service, Capitol Police and D.C. police crews responded to the scene. One bomb squad brought an X-ray scanner, another an advanced cutting tool and a third a videoscope that revealed the contents of the drum -- a concrete block like those used to post signs around construction sites.

"If it can be detected," Capitol Police Deputy Chief James P. Rohan said, "we should be able to detect it."

For all their work, however, officials worry about a missing link in the preparedness chain: the public. Some planners fear that panicked citizens far removed from an area of attack will storm hospital emergency rooms or clog highways, impeding rescuers and other personnel.

Gainer said he is less worried about weapons of mass destruction than about the long-term psychological impact of a conventional suicide bomb. He and other officials consider the risk of a chemical, biological or radiological attack to be relatively small, given the difficulty of obtaining the necessary material, handling it safely and converting it into a deadly weapon.

"If we saw what a couple of shooters could do to this area last year," he said of last fall's sniper attacks, "then think about the reaction to a suicide bomber standing by a bus stop at a mall. I don't think the American public is psychologically ready to handle it."

Spreading the Word

Government itself has had a hard time communicating information to the public. Last month's duct tape and plastic sheeting advisory from Homeland Security officials turned into a public relations fiasco.

The fact remains that many individuals and businesses, even employers downtown, have done little to get ready for an emergency. Many companies have yet to develop plans for how they would maintain operations and feed, shelter and protect employees unable to leave the office.

"The professionals are in a strong but not perfect preparedness state," said George Vradenburg III, head of the Greater Washington Board of Trade's homeland security task force. "But the public -- and I would include the business community -- has only begun to get some information and is unevenly prepared at best."

At River Terrace Elementary School last Monday night, about 200 residents gathered to try to rectify that problem, in one of the first in a series of neighborhood preparedness meetings sponsored by the D.C. government. Their worries were reflected in their questions: They wanted to know about evacuation routes, whether schools could house and feed their children in a lockdown situation, what would happen to the elderly or the handicapped in the event of an attack.

Officials talked about keeping emergency supplies in their homes and cars and setting up a plan for communicating with family members. "Disaster volunteers" were recruited to receive Red Cross training.

"We want you to know what you need to know before, during and after an emergency," said Barbara Childs-Pair, deputy director of the D.C. emergency preparedness agency. "Because the better prepared you are, the better prepared we're going to be."

Area officials are trying to inform residents about security efforts, but "the public . . . is unevenly prepared at best," one expert said. Helicopter patrols by U.S. Park Police are part of the front-line effort aimed at preventing a terrorist attack in the Washington region. U.S. Park Police Officer Keaton M. Wright waits to take off at the heliport in Anacostia. Michelle Hall-McCoy, flanked by her daughter Malika McCoy and mother, Lillian Hall, asks a question during a meeting on community safety at River Terrace Elementary School in Northeast Washington.Helicopter patrols are only part of the security effort. Behind the scenes, area officials are trying to prepare a response to a range of dangers.