Nearly a decade has passed since Timothy McVeigh used a homemade bomb to blow up the Alfred P. Murrah Federal Building in Oklahoma City. After the smoke cleared, investigators determined that the April 19, 1995, blast had come from a bomb powered by ammonium nitrate (AN) and nitromethane, a device more commonly known as a fertilizer bomb. Although the Oklahoma City bombing raised international awareness of such bombs, they had long been — and continue to be — a favorite among militants around the world.
Irish Republican Army militants utilized fertilizer bombs for decades before their April 24, 1993, bombing of London's NatWest Tower, which killed one person, injured 30 and reportedly caused tens of millions of dollars in damages. The Ryder truck used in the 1993 attack against New York's World Trade Center was rigged with a fertilizer bomb. In October 2002, more than 200 people were killed by a series of fertilizer bombs in the Indonesian resort town of Bali. In November 2003, 62 people were killed in the simultaneous truck bombings of two synagogues and a British consulate in Istanbul, Turkey. Most recently, a British police raid on suspected Islamist radicals in March netted a half-ton of AN.
The challenge facing counterterrorism authorities worldwide is that the ingredients to make these bombs are inexpensive and readily available on the open market. Furthermore, the ubiquitous nature of AN makes it hard to trace to one particular source. Additionally, these powerful and extremely lethal bombs are relatively simple to construct — manuals are easy to obtain — and they are unlikely to detonate prematurely. For all these reasons, this type of bomb often becomes the weapon of choice for militants.
Although explosives-grade AN is tightly controlled — and obviously not sold in neighborhood farm supply stores — militants have easily overcome this obstacle. The most common fertilizer bomb combines AN with fuel oil, to make the compound ANFO. Commercial AN fertilizer is not sensitive enough to be sufficiently explosive, even with a blasting cap, as the fertilizer must contain about 25 percent AN or more to be effective. Fertilizer of lesser strength, however, can be processed into a form with higher AN content, and chemicals can be added to ANFO to make it explosive. A much easier route, however, is to use a "booster," a smaller bit of high explosive, such as C-4, which causes the ANFO to be sensitive to blasting caps. Some high explosive boosters can be mixed at home as well.
The velocity of an ANFO blast wave is slower than that of high explosives and these bombs do not have as much of a shattering effect (brisance). However, an ANFO bomb is still very powerful, and its heaving action can cause significant damage, which is why the compound is used extensively in mining and construction.
Another type of fertilizer bomb made with ammonium nitrate and nitromethane (ANNM) — which McVeigh chose — is less stable, but does not need a booster and can be fully detonated with just a blasting cap.
Because fertilizer bombs create such large explosions, such as the Oklahoma City bombing or the London attack, the havoc they cause is huge — another reason for their popularity. Although not as strong as their high-explosive brethren, AN-based bombs can be constructed on a much more massive scale. It is much easier to produce a thousand pounds of ANFO than obtain a thousand pounds of C-4. Conversely, ANFO is inefficient to use in small improvised explosive device attacks because the explosive booster material is enough to do the job. Indeed, large explosive attacks are more likely than not to be the result of fertilizer bombs because of their cost and their relative ease of assembly. Like all blasts, however, a fertilizer bomb explosion leaves traces that experts can use to immediately identify its source. A telltale sign of an ANFO bomb, for instance, are its signature residues and the lingering smell of ammonia following a blast. This helps law enforcement authorities determine the cause of the blast, but, because the odor is emitted after the blast, it does little to prevent such an attack. Some effort, though not much, has been made to attempt to restrict the sale of fertilizers and to otherwise control AN in Western countries. For example, Britain took steps to strictly control the nitrogen content of AN, though militants have been able to circumvent that effort by purifying the AN. There also has been some talk on Washington's Capitol Hill to add tags, called "tagents" to products such as AN so that they can be traced. Furthermore, fertilizer dealers in the United States have been asked to report suspicious behavior. Ultimately, however, law enforcement sources in the United States report that these ingredients are commonly stolen, even large amounts of it. And, they add, the thieves often get away. On the other hand, authorities are more aware of the potential risk from fertilizer bombs and pay more attention to events — such as fertilizer theft — which increases the odds of disrupting an attack. Law enforcement sources in the United States say they have relied on good police work, good tips — and luck — to prevent further fertilizer bomb attacks. Ultimately, preventing such attacks must be looked at in the context of stopping terrorist attacks in general. Efforts have been made to fortify buildings that are more prone to attack or to make design adjustments to minimize the deadliness of fertilizer bombs, such as installing windows that do not fling lethal shards of glass during an explosion. The Australian Embassy in Jakarta weathered such an attack in October. However, every building cannot be fortified, every crowd cannot by meticulously guarded and every bag of fertilizer cannot be tracked. Hence, the threat remains.