THE BOMB LURKING IN THE GARDEN SHED

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Ammonium nitrate is a double-edged chemical. Spread it on a field and it becomes a benign fertilizer that helps produce bumper crops. Mix it with fuel oil and it becomes the kind of deadly bomb that shook Oklahoma City last week. The U.S. makes 8 million tons of the white crystalline compound each year, and it is widely available in gardening centers, supply warehouses and at construction sites. But just because anyone can buy ammonium nitrate for about 11¢ a pound doesn't mean that anyone can make it explode at a particular time and place. The act of terror that demolished the Murrah building and destroyed scores of human lives required a basic understanding of chemistry, skill at bombmaking and some technical know-how to jury-rig a few key components that are not easy to get.

A blast that size also required a great deal of explosive material--4,000 lbs. by one estimate. If the terrorists bought ammonium nitrate in standard 50-lb. bags, that's 80 bags--a cumbersome load for sure, but one that would fit snugly into a midsize Ryder truck. If all the material detonated--and there is reason to believe it didn't--the explosion released more energy than the blast from a ton of dynamite.

Some 95% of all commercial explosives work done in the U.S. uses ammonium nitrate and fuel oil in specially prepared mixtures called ANFO, according to Jeffrey Dean, executive director of the International Society of Explosives Engineers in Cleveland. The combination is ideal for blowing rock out of quarries, mining rich veins of ore and laying roads. It's ideal not only because of its destructive potential but also for its relative safety. So stable that it cannot explode spontaneously, ANFO is considerably safer than dynamite, which contains volatile nitroglycerin. You can light a match to ANFO, jump up and down on it, even shoot a bullet at it, but it isn't likely to blow up. To unleash the power of ANFO, demolition workers must use a device called a detonator-which contains a small amount of a highly explosive material-and a powerful booster charge that will create enough of a shock wave to set off the compound. A special type of detonator called a blasting cap is most frequently used, and its sale is tightly controlled.

There are more than 800 different commercially available ANFO preparations. One of the more common mixtures uses pellets of ammonium nitrate mixed with diesel fuel to form a semiliquid that has a consistency of mayonnaise. Because of ANFO's stability, it is usually transported in heavy- duty trucks that often resemble cement mixers. Or construction crews can mix it on site by combining bags of high-grade ammonium nitrate with drums full of fuel oil.

Contrary to the impression left by some press reports, the use of commercially prepared ANFO is carefully monitored. People who want to buy it must produce a federal permit or a license issued by the U.S. Bureau of Alcohol, Tobacco and Firearms. They must also have a permit from the Department of Transportation to move the ANFO, and various state or local permits to use it. In addition, most of the people who buy and sell ANFO know one another. Any stranger who wanted to buy a large amount of ANFO would immediately arouse suspicion. Theft of any quantity would quickly be reported to the authorities.