As investigators hunt for the culprit behind the explosive devices targeting top Democrats, CNN and actor Robert De Niro, the mystery evokes memories of America's "most prolific bomber" -- the notorious Unabomber -- who was so methodical in his attacks he went nearly 20 years without being captured.
The first device by the Unabomber -- later revealed to be Ted Kaczynski -- was primitive and homemade, exploding at a Chicago university in 1978, according to the FBI.
His bombs then became "increasingly sophisticated" as he mailed or hand-delivered them to random victims, the FBI said. He even threatened to bomb planes.
The FBI formed a task force to investigate the terrifying mystery in 1979, calling it the "UNABOM" case, which stood for the "UNiversity and Airline BOMbing" targets.
His 16 bombs killed three people and injured two dozen others, the FBI said, until his arrest in 1996.
Hand-crafted without a trace of evidence
Kaczynski was so "methodical," said former FBI agent and ABC News contributor Brad Garrett, that he was able to build his bombs without leaving any trace of DNA, hairs or fibers.
He wore gloves and even vacuumed the bombs "to eliminate the possibility that you could find a trace of him on the package," Garrett said.
Many of his bombs were hand-crafted out of wood. While not as powerful as a metal pipe, that made his devices more difficult to trace since they were homemade instead of purchased, said Garrett.
He even tried to mislead investigators by writing initials that had no connection to himself inside his creations, Garrett said.
Bombs through the mail
The bombs Kaczynski sent through the mail were "intricate, tripwire-types," Garrett said, "which are not easy to build."
"He was so prolific," said former FBI agent and ABC News contributor Steve Gomez, because when he put bombs in the mail, they didn't explode while going through the delivery process. The bomb would only go off once the victim opened the envelope.
"That's truly amazing in the eyes of the bomb-tech community," Gomez said. That was "very, very sophisticated, which is why he's viewed as the most prolific bomber in America."
Patience and diversity
His patience -- and diversity in targets -- also helped him evade capture.
Kaczynski's attacks could be months, sometimes years, apart, instilling fear all over again once a new explosion hit.
He "didn't get enthralled in the rush," or let his ego get in the way, Gomez said.
And while he focused on academics, research and technology -- as he perceived technology to be the downfall of civilization -- he had no one specific target, Garrett said.
"He thought modern technology and industrialization were basically destroying society and taking away personal freedoms. People like him, they're acutely paranoid and disillusion," Garrett said.
Spending all of his time alone likely magnified that paranoia, Garrett added.
A big break
Years went by without a solid lead -- until 1995, when the Unabomber released a 35,000-word manifesto.
Investigators worried about giving in to his demands, but ultimately decided to publish it, hoping someone could name the writer.
David Kaczynski then came forward, giving authorities letters written by his brother, Ted Kaczynski.
FBI linguistic analysis found that the writer behind the manifesto and the individual who wrote those letters "were almost certainly the same," the FBI said.
Investigators also learned Ted Kaczynski grew up in Chicago, where the first bomb was, taught at the University of California at Berkeley, where two devices were left, and had lived in Salt Lake City, which was also a target.
By that point, Ted Kaczynski was living as a recluse in a small cabin in Montana, where he was arrested in April 1996.
"The fact that he moved out into this desolate area -- he wasn't on anybody's radar," said Gomez.
A live bomb and a "wealth of bomb components" were found at the cabin, the FBI said, as well as "40,000 handwritten journal pages that included bomb-making experiments and descriptions of Unabomber crimes."
Ted Kaczynski pleaded guilty in Jan. 1998 and was sent to a Colorado prison, the FBI said.
The manifesto "was his undoing," said Gomez.
Without that, Garrett added, "he may still be out there in that cabin in the middle of nowhere in Montana."