"There's been no other criminal like this that I know of."
— James Fox, dean of Northeastern University's College of Criminal Justice

By the time authorities arrested a suspect in April 1996, the Unabomber had become one of the most widely known serial killers in the United States — a man who eluded authorities nearly two decades while managing to kill three people, injure 23 others and disrupt a nation's air travel.

If defendant Theodore Kaczynski turns out to be the notorious bomber, he will have earned his place in U.S. criminal history. The 18-year search for the Unabomber was the nation's most expensive manhunt, costing more than $50 million.

A slow start: The bomber first struck in May 1978 at Northwestern University and again at the suburban Chicago campus the following year. Yet it wasn't until a third bombing, aboard an American Airlines flight from Chicago to Washington, D.C., in 1979 that the incidents were connnected.

The naming of a bomber: The perpetrator was initially called "The Junkyard Bomber" at the FBI Crime Lab because parts of the bombs came from household items such as bits of furniture, plumbing pipes and sink traps. That changed after the bombing of United Airlines president Percy A. Wood. The case, which now comprised two university-related strikes and two airlines-related explosions, became known by the FBI case code Unabom. A task force was established with agents from the FBI, the Office of the Postal Inspector and the Bureau of Alcohol, Tobacco and Firearms.

Avoiding capture: The bomber did not lick stamps used on his packages, left no fingerprints on his carefully crafted devices and built his bombs with the most generic materials he could find. He also gave no clue as to his motives for the explosions.

A fatal first: The Unabomber's 11th device killed Sacramento computer rental store owner Hugh C. Scrutton on Dec. 11, 1985. Investigators decided then to go public with their investigation, and the Postal Inspection Service offered a $25,000 bounty for the Unabomber's capture.

Going public: Shortly after Scrutton's death, the bomber sent a letter to the San Francisco Examiner detailing his anarchist objectives, taking responsibility for at least five bombings and disclosing that the initials FC etched on some of the bombs stood for the name "Freedom Club." However, this first attempt at communication failed. Editors at the newspaper said they had no evidence such a letter ever arrived despite the terrorist's claims, made in a 1995 mailing to Penthouse magazine, that it was sent.

At last, a break: A bombing outside a Salt Lake City computer store in February 1987 produced a description of the suspect. A secretary, the case's first and only eyewitness, saw a man in a hooded sweatshirt and aviator sunglasses leave what appeared to be a bag of wooden boards in the store parking lot. The device later exploded when moved by the store's owner.

Then silence: Perhaps shaken by the sighting in Salt Lake City, the bomber disappeared for more than six years. Investigators had several theories about the Unabomber's silence: He'd committed suicide, was serving time for an unrelated charge or was busy perfecting his technique.

The return: The bomber resurfaced in June 1993, possibly influenced by two headline-grabbing events that year — the World Trade Center bombing and the standoff at the Branch Davidian compound in Waco, Texas. Geneticist Charles Epstein and Yale computer scientist David Gelernter were injured in separate bombings when each opened packages mailed from Sacramento.

A letter to The New York Times, also mailed from Sacramento, stated its postmark would precede a newsworthy event, presumably the bombings. The writer claimed to represent an anarchist group called "FC" and gave an identifying number — 553-25-4394 — to ensure the authenticity of future communication. The nine-digit number was the Social Security number of a young Northern California man. He was ruled out as a suspect because he'd been in prison when some of the bombings occurred.

Capital stay: A desk clerk years later would recall Kaczynski's lodging several times, including a stay the same month bombs were mailed to Epstein and Gelernter, at the Royal Hotel in Sacramento.

A reward: In October 1993, the U.S. government posted a $1 million reward for the Unabomber's capture and set up a 24-hour hotline — 1 (800) 701-BOMB — which eventually fielded more than 20,000 calls.

A red herring? A letter in 1993 carried the almost imperceptible imprint of a message: "call Nathan R Wed 7 p.m." Authorities checked out some 10,000 Nathan R's in the United States but found no leads.

California criminal: By December 1994, seven of the 15 bombs fashioned by the serial bomber, including a device which that month killed New Jersey ad exec Thomas J. Mosser, had been mailed from or exploded in San Francisco, Oakland or Sacramento. Federal officials concluded that the bomber most likely lived in or had close ties to Northern California.

Whodunit: Early suspects once included a group of college students who belonged to a Dungeons & Dragons club. In January 1995, members of the San Francisco-based Unabom task force thought they'd finally found their killer, a blue-collar worker who spent significant time in his garage on woodworking and electronics projects. They pulled his phone records, interviewed neighbors and his employer, and followed the San Francisco Peninsula resident 24 hours a day. But later it became clear he was not the bomber.

Voice of a male: On April 23, 1995, the day before a bombing which killed timber lobbyist Gilbert B. Murray, a man with a gravelly, strained voice left a message on the answering machine of an insurance association in Sacramento: "Hi. I'm the Unabomber, and I just called to say 'Hi.'"

A deal: The following day, in a letter to The New York Times, the bomber offered to stop killing if his 35,000-word treatise would be published in a national periodical. The manifesto was sent to the Times, The Washington Post and Penthouse magazine that June. It was published on Sept. 19, 1995.

A college try: Prior to its publication, investigators distributed 50 or so copies of the manifesto to college professors in hopes that it would trigger an association with a former student.

Scouring the Sierra: Forest rangers and sheriff's deputies in remote areas across California searched in May 1995 for evidence that the bomber had tested his deadly devices on national forest land in the Sierra Nevada after the killer stated in a letter that he was growing weary of "searching the sierras (sic) for a place isolated enough to test a bomb."

Net help: That same month, the Unabom Task Force made a plea via the Internet asking users to share any information they had regarding the Unabomber.

Short-lived suspect: In October 1995, police questioned a man arrested for a traffic violation in Evanston, Ill., who they believed may have been connected to the case. A check had revealed that the California license plates on the van he drove did not match the vehicle, but the man was later dismissed as a suspect.

A brother's dilemma: After the manifesto's publication, David Kaczynski, who had noticed similarities between his reclusive brother, Ted, and the Unabomber profile, also saw a resemblance between phrases and ideas used in the manifesto and those in his brother's writings. After a private investigation confirmed the possibility that Ted was the bomber, David, through his attorney, contacted the FBI.

Montana manhunt: FBI agents, including members of its Hostage Rescue Team, who are trained to survive for long periods in the wilderness, poured into Lincoln, Mont., using infrared and satellite surveillance to monitor Theodore Kaczynski's cabin. On April 3, 1996, they obtained a search warrant, and Kaczynski was later arrested for possession of bomb components.

The search produced a bounty of evidence, including what is believed to be the original draft of the Unabomber manifesto, an original letter sent to The New York Times, the names of about 25 Berkeley math professors, three typewriters and a completed explosive.