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 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.
(Sarah L. Voisin -- The Washington Post)
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.