By the time federal authorities arrested Theodore J. Kaczynski (aka the “Unabomber”) at his primitive log cabin in Montana in April 1996, he had managed to outwit the law for more than 17 years.

From 1978 to 1995, the former math professor with a genius-level IQ and a massive grudge against modern technology had mailed or hand-delivered 16 homemade explosive devices to universities, businesses, homes and public areas across the United States, killing three people and injuring nearly two dozen more.

The desperate search for the Unabomber stands as one of the longest-running manhunts in U.S. history, eventually involving than 150 full-time investigators, analysts and other agents from the Federal Bureau of Investigation (FBI), Bureau of Alcohol, Tobacco and Firearms (ATF) and U.S. Postal Inspection Service. In the end, after an investigation lasting nearly two decades, it would be Kaczynski’s own words that led to his capture.

The Unabomber’s campaign of terror began on May 25, 1978, when a brown paper-wrapped package found on the campus of the University of Illinois in Chicago was returned to the supposed sender, a professor at nearby Northwestern University. As the professor had not mailed the package, he handed it over to campus security; it then exploded, injuring the security guard tasked with opening it.

By late 1979, two other bombs had exploded, including a second at Northwestern and one that exploded aboard an American Airlines flight bound from Chicago to Washington D.C. Another package bomb sent in early 1980 badly injured Percy Wood, the president of United Airlines. Aided by agents of the ATF and Postal Inspection Service, the bureau formed the UNABOM task force, named for the suspected serial bomber’s earliest chosen targets: universities and airlines.

Though they conducted exhaustive forensic examinations of the bomb components and made efforts to link the victims in order to recover clues to who the bomber might be, investigators came up empty. The bomber made his explosives from common scrap materials—including wood, fishing wire, nails and tape—that were widely available, and had clearly taken great care to leave no identifying trace behind.

As FBI criminal profiler James R. Fitzgerald told NPR in 2017, lab tests suggested that the Unabomber had torn the skins off the batteries he used and fashioned his own adhesive by melting down deer hooves. “And, of course, no fingerprints, no DNA—nothing like that," Fitzgerald added.

As the attacks continued, the bombs also became increasingly sophisticated and destructive. In 1985, the owner of a computer store owner in Sacramento, California, became the first of the Unabomber’s victims to die of his wounds.

The first major break in the case came in 1987, when a woman reported seeing a man in a parking lot outside a computer store in Salt Lake City moments before a bomb exploded there. From her account, sketch artists created the now-famous sketch of the suspected Unabomber: a man with a mustache, wearing aviator sunglasses and a hooded sweatshirt.

The bombings then stopped for six years, but began again in mid-1993 with attacks on university professors in San Francisco and New Haven, Connecticut. Two more fatal attacks followed in December 1994 and April 1995, only two days after the Oklahoma City bombing.

In June 1995, the bomber sent a 35,000-word “manifesto” to the New York Times, the Washington Post and other media outlets, railing against the Industrial Revolution and the evils of modern technology. After debating the wisdom of giving terrorists such a public forum, FBI Director Louis Freeh and Attorney General Janet Reno authorized its publication, reasoning that it could help lead to the bomber’s identification.

This proved to be the case, as a woman named Linda Patrik recognized the language in the manifesto as similar to that of letters her husband, David Kaczynski, had received from his estranged older brother, Ted.

"I thought I was going to read the first page of this, turn to Linda and say, 'See, I told you so,’” David Kaczynski told ABC News in 2016, of the moment his wife confided her suspicions. “But on an emotional level, it just sounded like my brother's voice.”

A precocious math genius raised in the Chicago area, Ted Kaczynski had won a scholarship to attend Harvard University at the age of 16 and, in 1967, became the youngest-ever professor of mathematics at the University of California, Berkeley. But just two years later, he left modern society behind to live in the woods, growing, foraging and hunting his own food.

After David Kaczynski provided a writing sample to the FBI that was highly similar to the bomber’s manifesto, agents arrested Ted Kaczynski near Lincoln, Montana on April 3, 1996. They found a wealth of incriminating evidence inside his tiny cabin, including another bomb, bomb-making components and the original manuscript of the manifesto.

In hindsight, yet another reason emerged for the investigation’s many dead ends over the years, this one based on the bomber’s unusual biography. Though computers had helped the FBI compile a vast list of potential suspects based on the targets and the locations of the attacks, and Ted Kaczynski’s name had even been on the list, investigators had believed they were looking for a man some 10 years older than he was.

“We felt strongly that his origins were in Chicago and that he gradually moved West," Jim R. Freeman, the top agent at the FBI's San Francisco office during the investigation, told the Times in 1998, shortly after Kaczynski received four life sentences for his crimes. “How could we know he went to Harvard when he was 16 years old.”