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Federal investigators were using sophisticated scientific analysis of metal fragments and other evidence found in the Oklahoma City rubble that could lead them directly to the killers.

Every bomb has a distinct "signature," and sometimes such evidence can even pinpoint the individual who designed or assembled the bomb or can help identify the specific terrorists, investigators and experts say.But they caution against expectations that such evidence bring the killers to prosecution. That is because of the widespread availability of the "fertilizer and fuel oil" bomb apparently used in Wednesday's blast.

"I call it the MacGyver bomb - you can make it out of almost anything," said Andy Armstrong, an Arlington, Texas, forensic chemist who specializes in analyzing evidence from bomb blasts and fires.

Armstrong, who analyzed the Branch Davidian compound fire two years ago, was referring to a popular 1985-1991 TV show in which the hero often fashioned amazing contraptions out of household components.

The Oklahoma City bomb is thought to have been manufactured from ammonium nitrate, a common fertilizer, soaked with diesel or motor oil, and perhaps mixed with additional blasting agents such as aluminum powder or dynitrotoluene to boost the power of the explosion.

Traces of these chemicals are analyzed using liquid chromatography, gas chromatography or mass spectroscopy, techniques that resolve chemical mixtures into their component compounds.

Federal agents said evidence were examined on site and then sent to the FBI laboratory division in Washington for the more detailed analysis.

If traces of raw ammonium nitrate power are recovered, it is possible to identify where it was manufactured, Armstrong said. That is because chemical plants reduce the chemical from liquid to powder at different rates, producing crystals of different sizes.

With that information, agents could begin to track down where the bags of ammonium nitrate were purchased.

In addition to chemical analysis, explosives technicians are taking precise measurements of the crater left by the blast.

"There's a simple mathematical equation you use to determine the power of the device given the composition of the ground it made the crater in," said a military demolitions expert who asked not to be identified.

"From that data, you can fairly precisely identify what kind of bomb it was, and you can begin linking it to terrorist organizations that have used that type of device in the past," he said.

Meanwhile, agents are searching the blast site for other evidence, including metal fragments from the vehicle believed to have held the bomb, pieces of the sacks that held the ammonium nitrate, and bits of plastic sheeting with which the bombers may have lined the vehicle to prevent fuel oil from leaking out.

While federal agents declined to discuss the specifics of this case, other officials with experience in tracing bombs to terrorist groups said such analysis often provides enough evidence to link an explosion with a specific terrorist group.

The FBI, CIA and other intelligence agencies, as well as foreign intelligence organizations, maintain extensive profiles of terrorist groups and previous bombings.

"It's a matter of reverse-engineering the bomb," said Armstrong. "You look at the components and you can recognize the thing. It's like a mechanic looking at a part and saying, 'Hey, I know this, it's from a 1935 Dodge."'

"A lot of times you can even recognize the individual bomb designer. You can tell if he's left- or right-handed by the way he solders connections," Armstrong said.



Bomb's ingredients readily available

A bomb like the one used in Oklahoma City can be made from supplies easily and cheaply bought at many garden supply stores. A few dozen bags of fertilizer - at about $8 per 50-pound bag - some diesel fuel and a blasting cap is all it takes. Total cost for a bomb big enough to destroy America's sense of security: $200.

The key ingredient in the bomb appears to be ammonium nitrate, a common farm fertilizer that is the choice of terrorists the world over. The chemical compound has been found in bombs used by the Irish Republican Army, by terrorists in Thailand and most recently in the United States in a bomb that killed New Jersey ad executive Thomas Mosser last year.

"You can buy plain old fertilizer right off the shelf and if you put enough diesel fuel on it, it'll modify it just right," said Norman Stephenson, a former FBI agent who helped track down a violent group of white supremacists known as The Order. "Just put a blasting cap in it and you've got a very explosive device."

Instructions for would be bomb-builders are also easy to get - from military manuals to a variety of how-to books such as "The Poor Man's James Bond" and "The Anarchist's Cookbook."