Read: 98 years of mail fraud

The Sayoc case doesn’t just highlight the difficulty of sending explosives through the mail without getting caught. It also shows the range of tools, from the basic to the technologically advanced, available to law enforcement in chasing a suspect like him. Some of those methods have improved significantly since the Kaczynski era. But Kaczynski himself was unusually sophisticated in concealing his tracks.

“We’ve been fortunate that mail bombs and mail packages involving these kinds of crimes are one of the easiest to solve,” said Terry Turchie, who headed the FBI task force that finally caught Kaczynski. Most such cases are minor, he said, and involve threats or devices, like Sayoc’s, that don’t work.

Even when they do work, Turchie said, mail bombs distribute pieces of forensic evidence, sometimes buried under charred furniture and piles of paper. Nails, pieces of pipe, glue residue, explosive material, batteries, maybe a detonator—all offer clues. Often, these clues can open a trail back to stores where the components were purchased—for instance, a specific batch of batteries being carried at a specific store—and from there to clerks who may recall details about who made similar purchases.

Then there’s the setup of the postal service itself, which has a limited number of distribution centers in each major metropolitan area, as well as its own inspection service, which investigates possible crimes using the mail system. Several of the bombs Sayoc allegedly sent, according to the criminal complaint, were processed through a single distribution center in Opa-Locka, Florida, which helped investigators narrow the search for a suspect to the southern part of the state.

There’s also the progress of investigative technology, including the proliferation of security cameras and the increasing sophistication of DNA analysis since the Kaczynski bombings. During the Unabomber investigation, Turchie said, there were not typically cameras outside of post offices, and certainly not ones that could run for 24 hours at a time.

It was the presence of such security cameras that helped solve another recent serial-bombing case, in Austin, Texas, this year, when cameras captured the suspect’s car leaving a FedEx outlet after a suspicious package had been dropped off there.

Read: What makes a serial bomber tick?

Kaczynski may have benefited from the fact that there were fewer tools available to law enforcement in the 1980s and 1990s, when he was conducting his attacks. But he was also nefariously adept at thwarting the forensic analysts on his trail. In order to remove his fingerprints, he carefully polished his bombs and sanded the wooden boxes he placed them in. He was known among some investigators as the “junkyard bomber” for his habit of fashioning bombs out of parts found in landfills rather than purchasing them in stores, the better to avoid leaving clues. Turchie recalled that more than one of Kaczynski’s bombs had avocado-colored nails—leftovers from the interior-decorating “avocado craze” of the 1960s and 1970s.