Finding the Unabomber was a classic case of modern detective work. Profilers spent countless hours trying to plumb the Unabomber's mind, hoping to determine his motivation, the source of his rage, the links between his victims.

While these questions hovered in the foreground, another, different type of investigation amassed a mountain of evidence from exhaustive -- and minute -- examinations of the debris left behind from each explosion.

During the course of the nearly two-decade probe, teams of forensic scientists discovered numerous similarities among the bombs attributed to the Unabomber, everything from the specific construction of each bomb to the small "FC" marking that was etched into eight of the devices.

Other similarities included the use of a wooden box or frame in all but one device, similar types of wiring and fuses and even the 3/4" black plastic tape used in every bomb.

Throughout the investigation, the FBI explosives lab continually put out missives to other bomb labs around the country, looking for similar devices or connections between bombs that showed a pattern. Responding to this forensic equivalent of a "Wanted" poster, other labs sent in devices that fit certain profiles.


The bombs had such unique "signatures" that only someone sitting over the Unabomber's shoulder would have been able to duplicate the work.


The hunt for the Unabomber was "like finding Lucy," says one source close to the investigation, referring to anthropologist Don Johanson's 1974 discovery of a humanoid skeleton that was a crucial piece in the puzzle of human ancestry.

But it was also a matter of understanding the bomber, and in this case, it became clear to investigators as the years passed that this one was obsessed with his devices. The attention to detail was truly unusual.

In fact, the bombs ended up having such unique "signatures" that only someone sitting over the Unabomber's shoulder would have been able to duplicate the work. Not only was the construction of the bombs unique, but other clues matched up, such as the stamps used on the packages and the "FC" stamp deliberately placed on the devices, presumably to make clear that the bombs came from the same person.

The "FC" stamp, for instance, was kept as an FBI secret until the late 1980s. After its existence was publicly announced, their lab reportedly received a few awkwardly constructed copycat devices with the "FC" mark, but their rickety construction made clear that they were fakes.

Tracking such bombs is complex science, drawing on multiple fields. But a bomb is almost as easy to evaluate after it explodes as beforehand. Since the bomber is usually more concerned with building the bomb than the science that will be used to solve the crime, they will often inadvertently construct bombs that leave significant clues behind.

"They have a singlemindedness about what they're doing," says Jay Siegel, professor of criminal justice and chemistry at Michigan State University. "They're not too concerned about the forensics or the criminology of the situation."

The Unabomber, however, was meticulous in covering his tracks: "obsessed" with the construction of his bombs, according to one law enforcement source, he was paying "a lot of attention, perhaps too much attention to what he was doing."

One of the first things a forensic scientist will look for when trying to identify a bomb is the chemicals used in the explosive charge. If a bomb squad is able to recover an undetonated device, the chemical analysis is easy. But even if a bomb goes off, most of the parts -- and the residue from the explosive charge -- can easily be recovered.

"If a bomb goes off in a living room, I will be able to go in there and bring back all the parts," says William McCarthy, a professor of criminal justice and sociology at Corning Community College and the former commanding officer of the New York Police Department's bomb squad.

The residue usually provides the first clues. Nitrates and chlorates are the usual suspects in most bombs (the Unabomber used ammonium nitrate, potassium chlorate and sodium chlorate). But these chemicals are frequently found in nature, so the location of the residue matters almost as much as its composition. Nitrates found on a cement floor can be useful; nitrates found in a patch of soil usually aren't.

Additionally, a bomb can often be identified by examining its other components. Sometimes a bomber will use commercially available parts to construct his bomb. Not the Unabomber's devices.

The use of everyday materials and homemade parts made his bombs hard to trace to any particular origin.

"They were not easily traceable because he fabricated the components of the bombs himself," says McCarthy, who attended UNABOM briefings during his time with the NYPD. "He didn't go into the local hardware store."

But investigators can also match up physical bomb components other than the charge by examining their precise chemical composition. A single batch of metal pipe, for example, may contain the same proportions of metals and the same impurities, says Siegel. If two bombs have parts that match chemically, they've probably come from the same bomber.

It's also important to consider why someone will use a certain type of bomb.

Pipe bombs are somewhat distinctive as bombs go. Unlike a higher-tech item like plastic explosive, which will detonate when set off with a fuse or blasting cap, pipe bombs rely on standard materials for their explosions -- or rather their "deflagrations," a rapid burning of the flammable material much like lighting a match. The charge in a pipe bomb requires some sort of initial combustion -- "heat, spark, friction or flame," says McCarthy -- to explode.

Of course, no explosion will happen without a switch to set it off. The Unabomber dealt in bombs that were "passive", which is to say they relied on the victim to set them off. Each bomb had a simple, but clever, device built in for that purpose.

"It doesn't take much to create a switch," says Siegel. "It is as simple as closing an electric circuit or opening it."

Mail bombs usually work because the switch has somehow been integrated into the package so that opening it will trigger the switch and set off the fuse. Many mail bombs are constructed so that they will be relatively stable until the switch is set off.

As for the Unabomber, the switches were triggered upon opening a sealed package, lifting a lever, or in the case of 1985 victim John Hauser, opening a three-ring binder. When Hauser lifted the binder's cover, it triggered the bomb inside and cost Hauser his right hand and part of his vision.

Other victims, Thomas Mosser, Hugh Scrutton and Gilbert Murray, paid with their lives.

But while scientists were able to demonstrate a long and intricate pattern of bombings, their findings didn't lead investigators directly to a suspect. That only happened when David Kaczynski, the alleged bomber's brother, read the Unabomber manifesto and called the FBI on his brother Ted.