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Before murdering thousands at the World Trade Center, Mohammad Atta made repeated trips to rural airports, trying to learn everything he could about crop dusters. Why? Most people think he was interested in spreading chemical or biological terror over a major city. But there is no evidence that al Qaeda had stores of such materials. Searches in Afghanistan turned up no major chemical or biological facilities-nor is there evidence the group somehow had access to stockpiles in the United States. New facts amplify the mystery. A crop duster manual was found in the possession of Zacarias Moussouai, who prosecutors think would have been the 20th suicide hijacker, if he hadn’t been arrested first. A federal witness, Essam al Ridi, testified at U.S. trial of the four men accused in the 1998 U.S. embassy bombings in Kenya and Tanzania that Osama bin Laden wanted to buy a crop-dusting business.

Why? Maybe bin Laden just wanted to dust crops. (I know nobody takes that one seriously.) Maybe he actually did have stores of chemical or biological agents, and we’ll uncover them soon. But I bet they don’t exist. I have a totally different interpretation of al Qaeda’s interest in crop dusters. Let me lead you to my conclusion by starting with a review of what happened on September 11-and more importantly, what didn’t.

When Mohammad Atta boarded American Airlines flight 11 in Boston that morning, the only illegal things he carried were his intentions-no guns, no explosives, no long knives. Despite all the well-documented weaknesses of our airline inspectors, the risks of being caught with a weapon were too great for Atta et al. to take that chance.

The brilliance of the operation was its low risk. No purchases of explosives. No illegal weapons. Virtually no infrastructure needed. The danger of someone finding out was nil, since even most of the terrorists didn’t have to know the mission. (And many of them may not have known right up to their deaths, despite bin Laden’s subsequent assertion that they were told just before they boarded.) Atta’s plan depended on the airline policy that directed pilots to cooperate with hijackers. Don’t argue; don’t threaten; just do what they ask. This approach had saved lives (and airplanes) in the past.

Atta and his co-conspirators took early flights, minimizing the risk that the flights would be behind schedule and making it easier to attack New York and Washington simultaneously. But even more importantly, they took transcontinental flights. That was to make sure the planes were fully loaded with fuel.

Gasoline, mixed with air, releases 15 times as much energy as an equal weight of TNT. That fact astonishes most people. The numbers are simple: one gram of TNT releases 0.65 kilocalories of energy; one gram of gasoline, mixed with air, releases 10 kilocalories. TNT is not valued for its high energy, but rather for its ability to deliver the material-shattering force that accompanies rapid energy delivery (high power). Even chocolate chip cookies have nine times the energy content of TNT. To tear down a building, if you’re in no hurry, don’t use TNT; hire some teenagers, give them sledgehammers, and feed them cookies.

The history of gasoline as a weapon dates at least to the Molotov cocktail of the 1930s. In World War II, what the flame throwers really threw was burning gasoline. Vietnam made gasoline-based Napalm famous. Recently, in Afghanistan, the U.S. killed and demoralized Taliban troops with “fuel-air” explosives. Why were these so dreadful? Because 7 tons of gasoline, mixed with air and detonated from a parachute, release the energy of over 100 tons of TNT. So don’t drop TNT if you can drop gasoline. It gives 15 times the bang per ton.

Atta and his terrorists hijacked a Boeing 767 that carried 60 tons of fuel, equivalent to 900 tons of TNT, nearly a kiloton. When it hit the north tower of the World Trade Center, it didn’t all explode, but it continued to burn for over an hour. A long slow burning can be more terrible than an explosion. The building’s structure finally failed, the columns buckled, the upper floors fell like a pile-driver on the lower floors, and they collapsed in turn. A near-identical attack felled the south tower.

Atta knew that September 11 would be the last day on which an airplane could be hijacked with ease. After September 11, sky marshals are hardly necessary. No pilot ever again will willingly hand over the controls to a terrorist. Even if a hijacker kills the pilots, the courage and the fury of the passengers and crew will be unleashed.

An Air Tractor 502 Crop Duster airplane is far smaller than a 767, but it is also a flying tanker. It has fertilizer containers that hold roughly 1,200 liters of liquid, plus a 500-liter fuel tank. It flies close to the ground, where it cannot be seen by most radar technologies. Fill ‘er up with 1,700 liters of gasoline, and you are carrying roughly 2.1 to 2.4 tons, the energy equivalent of 32 to 36 tons of TNT.

What could a single suicide pilot do with a full crop duster? Crash into Yankee Stadium during the World Series, or into the Superbowl, or the Olympics opening ceremony. The deaths, including trampling, might exceed those at the World Trade Center, with everything broadcast live on international TV. (I virtually held my breath during these recent events.) Or the pilot might target a petrochemical plant, or a nuclear waste facility near a large city. (Can we please move those radioactive wastes to the relative safety of Yucca Mountain in Nevada?)

Fortunately (for us), it turns out that the Air Tractor is notoriously difficult to fly, particularly when fully loaded, and even more so if kept at very low (radar-avoiding) altitude. Maneuvering a Boeing 767 is probably easier, as long as you don’t have to land. Moreover, the crop duster community is very close, small, and wary. Even before September 11, they didn’t let Atta photograph their planes or even sit in the cockpits. For the next few years, you can be assured that every suspicious visit to a crop duster facility will be reported immediately to the FBI.

But don’t take too much comfort in the difficulty of obtaining and flying a crop duster. Other kinds of small planes can be used. At the World Trade Center, jet fuel, a kerosene-based equivalent to gasoline, killed more people and destroyed more property than bin Laden had dreamed possible. Gasoline is a low-risk explosive, and doesn’t require any special license to buy. So beware. If any al Qaeda agents are left in the U.S. and they want to kill and terrorize, then their weapon of choice may well be one that you can buy at the corner station.

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