Year after year and death after death, investigators looked everywhere, from the checkout records for certain library books to the computer files of known criminals, from the storefronts of scrap metal dealers to the employee lists of companies dealing in artificial human limbs.
They took each clue they came across _ no matter how small, no matter how dubious _ and wrung as many leads out of it as they could. They picked the minds of experts in fields running the gamut from handwriting to forensic psychiatry.
At one point, appealing to the public, they set up a toll-free telephone number with a chilling suffix: 1-800-701-BOMB. At another, appealing to the unknown, they actually consulted a clairvoyant.
By the time the search apparently came to an end Wednesday, 18 years after it began, with the capture of a man believed to be the Unabomber in a remote, self-made cabin in the Montana wilderness, it had become one of the most extensive, intricate and endlessly frustrating manhunts in the history of the Federal Bureau of Investigation.
In 1993, after a series of recent attacks by the Unabomber, Christopher Ronay, the FBI's top explosives expert, said he had never faced a more challenging case in 22 years with the bureau.
"He is creative," Ronay said. "And he's elusive."
The effort to find him combined old-fashioned detective work with newfangled technology and drew on the formidable financial reserves of the federal government and the considerable acumen of some of the best criminologists in the business.
Thousands of people were interviewed, and three different federal agencies _ the FBI, the Bureau of Alcohol, Tobacco and Firearms and the Postal Inspection Service _ were involved in the pursuit.
Investigators moved from Washington to Salt Lake City, from San Francisco to Chicago, as they targeted a killer who left a string of hints, sometimes on purpose, that were as cryptic and tantalizing as those imagined by the writers of popular crime fiction.
Every few years, it seemed, there was a fresh tidbit, small but promising. And every few years, there was a fresh stab of frustration, as a man pursued by legions of the smartest, most experienced investigators in the country somehow managed to remain as mysterious and elusive as an apparition.
"You keep chewing on the same thing, hoping to go further, but it's just not there," John Killorin, a spokesman for the Bureau of Alcohol, Tobacco and Firearms, said in 1993, when several recent attacks by the serial bomber yielded new clues and a reborn hope of his capture. "Ultimately, you reach the point where there is no more work to be done."
So investigators would wait and hope and worry.
Ultimately, some of their best leads came only last year, from the long manifesto by the Unabomber that was published in the Washington Post on Sept. 19.
And the publication of the manifesto was apparently a crucial part of Wednesday's breakthrough, enabling family members of the man believed to be the Unabomber to grasp the significance of some old writings of his that they came across. These writings seemed strikingly similar to the Unabomber's other communications. Through an intermediary, the family contacted the FBI.
From the beginning, in 1978, the Unabomber made investigators' jobs difficult. He fashioned his own bombs, usually using such common materials as scrap wood and lamp cord, so that nothing about the devices could be traced.
But as if to taunt those trying to track him, he often left a signature on the devices that survived the explosions _ the initials FC.
For most of the first decade of the manhunt, the three federal agencies involved operated separately, a situation that some critics believe hampered their efforts. One of the few big potential breaks came when a witness saw a man believed to be the Unabomber.
This happened in 1987, when a man believed to be the Unabomber deposited a bag containing a bomb in the parking lot of a computer sales company in Salt Lake City. The bomb maimed a man employed by the company.
An eyewitness said he had seen the man who left the package, and from that witness' description came the crude, widely circulated sketch of the Unabomber as a mustachioed man in sunglasses and a hood. The witness further described the bomber as a white man in his late 20s who stood perhaps an inch or two under 6 feet and had reddish-blond hair.
This lead, like so many others, did not take investigators far, but some believed it was the reason the Unabomber ceased his campaign of terror for the next six years.
After the Unabomber struck twice in June 1993 and also mailed a letter to the New York Times, investigators had a new set of clues _ these missives had featured postmarks from Sacramento, Calif.
By carefully analyzing a piece of paper in one of the Unabomber's communications, investigators detected traces of a piece of writing that was otherwise invisible to the human eye. It spelled out a message of uncertain significance _ "Call Nathan R wed 7 pm" _ that sent investigators on another elaborate computer and paper chase.
Investigators also determined some of the materials used in packaging the bombs were similar to materials used for artificial human limbs, and wondered if the Unabomber had worked for such a firm.
Toward the end of 1993, in gestures that bespoke their waxing frustration, investigators announced the toll-free number and offered a $1-million reward for information leading to the apprehension of the Unabomber.
The search grew more urgent as the Unabomber's makeshift explosives became more powerful, and investigators increasingly concentrated their efforts in the Northwest, where they believed the Unabomber was living.
In 1995, investigators canvassed California scrap metal dealers and paid visits to leftists and environmental radicals living in the San Francisco Bay area.
One person they interviewed at the time, Garth Smith, said in August, "If they were looking for someone like me, they must not have a clue about who this guy is."
In another few months, at long last, they would.