Government bomb technicians have packed Chevrolet sedans, Dodge vans and Ryder trucks with 10 tons of explosives and have blown them up in the desolate New Mexico desert hoping to analyze the flight of debris over the sand.

Federal agents in Front Royal, Va., have trained more than 400 Labrador retrievers to sniff out the chemical compounds used in 19,000 separate explosives formulas.

Law enforcement officers have left thousands of calling cards across the country -- from a farmer's co-op store in McPherson, Kan., to a chemical company in West Haven, Conn. -- asking sales managers to report unusual interest in fertilizer or other components of homemade bombs.

The United States has spent more than $1 billion on these and other efforts to stop a single threat: the explosion of a car or truck bomb at a government installation or other structure. But 11 years after Muslim extremists used an explosives-laden van to attack the World Trade Center and nearly three years after the Sept. 11, 2001, terrorist attacks, even senior federal agents acknowledge that the country has virtually no defense against a terrorist barreling down the street with a truck bomb.

"If a person doesn't care about dying, they can pull right up to a building, push a button and the building would go," said Michael E. Bouchard, assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives. "That's why we have checkpoints and try to keep large vehicles away from buildings."

The government has been racing to devise ways to systematically detect and warn against plotters creating truck bombs. But those efforts are embryonic at best, government officials say, even as al Qaeda and other Islamic extremists have used the truck bomb time and again overseas and the threat to use it here is growing.

The frustrating struggle to thwart terrorists' low-tech, low-cost weapon of choice provides a case study of America's challenge in waging the fight in the post 9/11 world -- a fight in which the enemy is hiding and the traditional role of soldiers and weapons takes a back seat to intelligence and prevention.

It is a war in which the United States, with all its technological and economic advantages, has been unable to develop protection against a self-taught bomber assembling large amounts of explosives in secret, acquiring a vehicle and fading into the landscape before detonating a payload.

Since the 1993 World Trade Center bombing, the government has hardened federal buildings and military facilities at home and abroad; passed laws restricting the sale of explosives and shipments of hazardous materials; inspected thousands of people who deal with explosives; and researched explosive-detection and vehicle-disabling technology. But the only foolproof defense was on display last week, when heavily armed police sealed off buildings, roads and bridges in Washington, New York and Newark after the government issued an elevated terror alert focusing on five financial institutions.

The threat of truck bombs underscores the ways terrorists can turn America's economic strength and freedoms against itself, academic experts say.

"The challenge is to provide a level of security that does not impede normal life and commerce, which would achieve the terrorists' aims without even launching an attack," said Bruce Hoffman, author of "Inside Terrorism" and head of the Washington office of the Rand Corp., a nonprofit think tank.

In a society based largely on the free movement of information, bombmakers acquire their expertise from chemistry texts, the Internet or each other. Taking advantage of a giant economy that depends on efficiency, they can buy or steal bomb components and obtain vehicles without fear of regulation while security measures are resisted by many farmers, truckers, city planners and citizens. They exploit free movement of people through states and cities, requiring society to undertake extraordinary surveillance and spend large amounts of time and effort to find them.

"What do you do when you have whole cities built up with no regard to this threat?" asked Daniel Benjamin, former counterterrorism director at the National Security Council. "Are we going to turn Lower Manhattan into a pedestrian zone?"

Counterterrorism experts say the threat is especially striking because al Qaeda and other Muslim extremists have demonstrated mastery of the weapon. Since the first World Trade Center attack was plotted by Ramzi Yousef with 1,200 pounds of chemical explosives tied to Casio watch timers in a rented Ford van, al Qaeda cells perpetrated simultaneous truck bombings of U.S. embassies in Kenya and Tanzania in 1998, blew up three housing compounds in Saudi Arabia in May 2003, attacked resorts in Bali and Jakarta and carried out multiple bombings in post-war Iraq.

In Britain, authorities recovered a half-ton of ammonium nitrate in March, and in April, Jordanian officials disrupted a plan that involved tons of commercial fertilizer and two heavy trucks.

"The truck bomb is a pervasive threat. Al Qaeda is adept at it and comfortable with it, and for all those reasons it is difficult to protect against it," Hoffman said. "The lesson of September 11 was there's not a moment to lose, but we're constantly behind the curve. . . . We improve security, and it slows them down slightly, but it doesn't stop them."

A Strategy Shattered

On April 19, 1995, disillusioned Persian Gulf War veteran Timothy J. McVeigh and Army washout Terry L. Nichols blew the face off the Alfred P. Murrah Federal Building in Oklahoma City with a 5,000-pound mixture of ammonium nitrate and fuel oil, killing 168 people.

The bomb was instructive in its power and ease of assembly. Equivalent to 4,100 pounds of dynamite, the blast damaged 312 buildings, cracked glass as far as two miles away and inflicted 80 percent of its injuries on people outside the building, up to a half-mile away. ATF officials had never studied the effects of a vehicle bomb larger than about 1,200 pounds, an ATF explosives expert said.

The components came largely from a Kansas co-op. Nichols bought two tons of fertilizer in 50-pound sacks starting seven months before the attack. McVeigh also was careful to avoid detection, renting a Ryder truck from a Junction City, Kan., body shop one state away from his target.

Today, it remains difficult to detect similar activity. Nearly 5 million tons of ammonium nitrate fertilizer are sold each year in the United States. None of it is regulated, although its explosive properties are used in mining and construction and by armies around the world. Government controls are resisted by farm and chemical lobbies, who say they would burden law-abiding citizens and not thwart terrorists. U.S. law permits farmers to mix it with fuel oil for personal demolition uses.

Controlling vehicles is similarly problematic. There are 23.8 million trucks used for business purposes in the United States and 70 million more in personal use, according to the American Trucking Associations.

Unlike commercial aviation, motor vehicles are not registered by a single federal agency; they're not based at a fixed number of airports or operated by a small number of companies controlling access to them. There are 600,000 trucking companies, which have 2.6 million tractors, 3.1 million big-rig drivers and 5 million trailers, the association said.

Regulation is complicated not only by sheer numbers, but also by fragmentation of the industry and of state and federal regulators, analysts said. For example, unlike many countries in Europe, which have national motor vehicle databases, each U.S. state maintains its own records.

Also, 92 percent of trucking companies are mom and pop operations with 20 or fewer trucks, said Tom Nightingale, spokesman for Schneider National Inc., the nation's biggest truck carrier. Schneider holds just 4 percent of the market.

There is also the problem of rentals. The FBI and Department of Homeland Security issued their latest threat bulletin Thursday, warning car, truck and limousine rental companies to report suspicious people. "There is no standard type of vehicle associated with" delivering a car or truck bomb, the alert says.

The bulletin listed suspicious behavior and urged companies to file detailed reports. It took special note of limousines, which it said have larger storage capacity and may get special treatment to approach buildings.

And with 1 million cars and trucks stolen in the United States each year, counterterrorism agents say they would investigate only if other evidence linked it to terrorism.

With such challenges, law enforcement authorities say they have few warning signals to stop bombers from building their weapons and approaching their targets.

As one ATF explosives expert said, "The only true defense is to shut the road down so no one can come down there. Sedans, sport-utility vehicles, a Ryder truck, a large flatbed vehicle or a truck -- there's no sure-fire way to look at that vehicle and say, 'That's a large vehicle bomb.' " The expert spoke on condition of anonymity because of agency security rules.

Added Bouchard: "Distance is our friend."

For the U.S. government, blast walls, barricades and setbacks at sensitive buildings have become the last line of defense. The Pentagon, White House and Capitol increasingly resemble fortresses. Defensive measures costing hundreds of millions of dollars are proposed or underway at more than 20 facilities, and the government has adopted a 100-foot setback as a guideline for high-security new construction in the United States and overseas.

The problem is that hardening some locations might redirect terrorists to "softer" ones, including hotels, malls or stadiums, analysts said.

"You cannot secure all of the potential targets for the U.S. government or government employees in Washington, or New York City for that matter," said Ronald K. Noble, who was U.S. Treasury undersecretary when the Secret Service shut Pennsylvania Avenue in front of the White House in 1995 and is now secretary general of Interpol.

Michael Mason, assistant FBI director for the Washington field office, likened the sense of vulnerability to boxing in the dark against a terrorist with "night vision goggles. They know when they're going to attack, how they're going to attack and where they're going to strike," he said. "You reach out and think you have an elbow. You think you have a shoulder, but it takes time to put it all together to effectively strike back."

Competing Priorities

For four years in the 1990s at the White Sands Missile Range in New Mexico, government technicians wired vehicles with explosives and tracked the blasts' effects with high-speed cameras. In Florida and overseas, scientists conducted similar tests -- adding buildings to the destructive mix.

Dubbed "Dipole Might" and funded by the National Security Council, the tests mapped the flight of debris as small as a matchbook, crater patterns and even the street sign-bending effects of blasts. Those experiments have become the basis of U.S. truck bomb forensics, allowing investigators to identify the type and quantity of explosive from studying the effects of the blast.

But in 2000, the money ran out, and so did the tests, the ATF said. Agents say they need more. Based on the experience in Iraq and around the globe, the diversity of explosives has grown.

The desert tests reflect both the promise and the limits of the struggle to manage the threat. Some outside observers say other government efforts have not been creative or energetic enough.

"The administration has focused primarily on two areas. . . . One is aviation security, and the other is bioterrorism," said Benjamin, a former Clinton administration official and co-author of "The Age of Sacred Terror." "Truck bombs have been very far down the list."

In March, the Transportation Security Administration awarded a $19 million grant to American Trucking Associations to expand Highway Watch, a computerized instant-reporting network through which professional drivers and highway workers can report accidents, thefts, hazards and suspicious incidents nationwide.

Cited by TSA officials as a major initiative, it, too, was funded at half the $43 million the industry requested back in 2002.

"We are the point men. We are the Distant Early Warning line for the trucking security problem," said Jeff Beatty, security consultant to ATA, comparing the system to the nation's northernmost radar defense line during the Cold War to detect a Soviet nuclear attack.

Regulatory initiatives have been delayed or watered down because of concerns by industry groups that say a cure may be worse than the illness. In June, Homeland Security announced it had completed background checks of 2.7 million commercial driver's license holders authorized to haul hazardous materials, but it culled only 29 with potential terrorist connections.

Another fingerprint-based background-check program, which has been opposed by truckers, has been delayed nine months. The program, now scheduled to begin Jan. 1, would require states to collect fingerprints from hazmat drivers to undergo FBI checks as well, part of a USA Patriot Act requirement.

"People ask, 'What's the big deal?' But a one-hour delay [for the nation's truck drivers] costs the entire truck industry $500 million," Nightingale said.

Similar sensitivity limits the controls of bomb components. Last month, the fertilizer industry urged ammonium nitrate sellers to voluntarily track sales and require buyers to show identification. But it resists any government regulation, and only Nevada and South Carolina have laws requiring tracking.

The move followed a history of voluntary initiatives. In 1996, the Fertilizer Institute and ATF unveiled a "Be Aware for America" campaign after the Oklahoma City bombing, distributing 30,000 brochures and asking industry members to report suspicious activities.

In 2001, they launched another education campaign, "Be Secure for America," encouraging manufacturers, distributors and retailers to prevent theft. In April, after the arrest of alleged terrorists in England, ATF met again with industry officials and rolled out "America's Security Begins with You." This time, the mission was to raise awareness and ask for voluntary reporting of thefts or unexplained losses.

Kathy Mathers, spokeswoman for the Fertilizer Institute, said most fertilizer is sold in rural outposts. "These retail outlets and employees know their customers. A customer they don't know . . . will raise suspicions," she said.

On another front, the government last year began requiring all people receiving explosives to obtain a permit from the ATF -- "a major change," said Audrey Stucko, chief of the ATF's firearms and explosives services division.

The Safe Explosives Act requires users and sellers of explosives to submit photographs and fingerprints and undergo criminal background checks. About 12,300 licenses and permits have been issued by the agency.

At the end of the day, the nation's security experts say they expect terrorists will get their hands on the weapon and that keeping bombers away from buildings is their best hope.

The FBI's Mason, whose office is handling about 800 terrorism cases, warns that the public is "being fed a false bill of goods" if it is led to believe that every terrorist will be stopped. He described security measures as a "net that stretches from coast to coast" and government efforts as an attempt to "shrink the mesh."

"Despite all the reforms and changes being made at the FBI and other agencies, the best we can hope to do is shrink the size of the mesh, allowing fewer things to pass through," Mason said.

Staff researchers Madonna Lebling and Don Pohlman contributed to this report.

Government technicians blow up a vehicle in New Mexico. Mapping the spread of the blast's debris will help forensic scientists understand the type and amount of explosives used in other bombings.

A D.C. police officer finishes inspecting a truck near the IMF building in Northwest Washington. Last week's orange alert bulletin noted that "there is no standard type of vehicle associated with" a car bomb and urged special attention to limousines as well, which often get close access to buildings.