There's no cooler job title: Chief Explosives Scientist.

The person who holds it, the FBI's Kirk Yeager, spent decades in the field, examining tiny pieces of evidence, most of them partially destroyed, and working backward until he had a bomb, a bomb location, and, more often than not, clues that led to a suspect. "Imagine if you dump out a thousand-piece puzzle, throw out half the pieces, and light the other half on fire," Yeager says. He's good at burnt puzzles.

Yeager is matter-of-fact, telling stories about a bomb site in Beirut as if he were recounting a trip to the pharmacy. And he has a good sense of humor, even for a guy who doesn't wear a lab coat every day. Like when he talks about briefing former FBI director Robert Mueller, who hated any suit that didn't include a white oxford and a tie. Yeager wore a scarlet shirt and a white bolo. "I spent six years in New Mexico," Yeager says. "It's the tie I prefer." He looks mischievous as he says it, but that's probably just the effect of his eyebrows, which curl up like the tips of a hipster bartender's mustache.

"Every bombing is the same thing. You're walking into chaos."

Among the things he keeps at his desk are a device called a hell box—the plunger used by nearly every cartoon character about to blow something up with dynamite—and the model he made of a pipe bomb from the Boston Marathon bombing. He's proud of the replica, and patient as he describes the components (ball bearings, a fuse, gunpowder) and how it would work. Yeager's curiosity started as a kid, he says. "I've always been obsessive. There's a reason for things, and I like to know what that is." This work gives him the chance to push the limits of what normal chemistry and physics do. "We have to understand explosions in microseconds," he says. We asked him to explain what that feels like:

"Every bombing is the same thing. You're walking into chaos. You don't know what's in front of you, you don't know where the path is going to lead you, and as you start pursuing different avenues and closing down idle speculation that's wrong, and finding forensic clues, you start developing a fuller and fuller picture.

"What really sticks in your head is not what you see. I'm used to seeing chaos and carnage. But couple that with the oppressive heat and the smell of burnt diesel fuel and decay of bodies and all sorts of unpleasantness like that, and it's overwhelming. All your senses are engaged, basically the minute you hit a scene. One time, there were cars and chaotic traffic and noise, beeping horns. Just a flux of activity all around me. But the streets were closed off around the scene, and as I went closer and closer, the noise receded into the background and it became quieter and quieter. There's a reverence to every scene because these people have been shocked to their core. It takes on a very serious gravitas, and so getting into the scene is very quiet. We almost whisper to each other.

"If you've never been into a true chemistry lab, it's hard to describe what we set up in the field. There's all these instruments that are designed to do chemical analysis. They look like massive microwave ovens with tubing and tanks attached to them. There's gas chromatography, there's infrared spectroscopy, there's mass spec analysis—the things that we use to identify chemicals.

Yeager has worked many of the most notable terrorism bomb attacks in recent history, like Bali, October 12, 2002; Beirut, June 2, 2005; and Boston, April 15, 2013, seen here.

"You start doing swabs of things. After a bomb goes off, it sends residue everywhere for the main charge. Explosives aren't 100 percent consumed in any event, so if, for example, a bomb goes off in the street, you find nearby street signs and you swab them. At the most rudimentary level you take a pair of tweezers and a sterile cotton swab, and you sit there and rub it over the surface area and collect soot. And then you take that back to that suite of instruments and you extract that swab with water or maybe acetone to pull the explosive materials off and then you run it through your instruments to see what the bomb deposited. Part of what you're trying to figure out is the main charge. To determine that, you use the residue that's left behind.

"You're not so much using fancy equipment as you're using years and years of training and an understanding of explosives. Because a bomb can consist of anything. Once, we had this bar and we knew a bomb went off in it, but we didn't know if it was a suicide bomb or if someone threw a bomb in from the street, because we had conflicting witness reports. Another bomb guy and I were trying to figure out where the epicenter of the blast was. That's the first clue. This bar had a whole bunch of pillars. Imagine you're in the room with a bomb and there's a big support pillar. Where would you want that pillar to be in relationship to the bomb? You'd want to be behind it, because the fragmentation is going to be blocked by that pillar. So if that pillar is in the middle of the room, and you have a whole bunch of frag strikes on the wall behind it, you can tell where the sheltered area is. We got a whole bunch of twine and a whole bunch of sticks, really long sticks, like six or eight feet long. If you have enough pillars, and you bring string from the sheltered area past those pillars, you can actually zero in on the epicenter of the device. We figured out that basically the device was three feet off the ground. We saw no crater in the concrete, so that meant it was sitting on top of a table or was worn like a backpack. Someone didn't throw it in.

"They look like massive microwave ovens with tubing and tanks attached to them."

"At another bombing scene on a fairly quiet street, a portion of the curbing was totally destroyed where the bomb hit it. It was obvious where the bomb was sitting. I'm looking around and I see trees, trees with scars on them. It looks like something is penetrated in. So I get out my knife and I dig around these scars and start pulling out fractions of wire. Now at this point, I know that this bomb is probably an electric fusing system just from that, because I got wire, I got a conductor in there. There's always a setup to a bomb: Someone could actually do a chemical initiation, someone could light a fuse like Wile E. Coyote, someone could call into a phone, there could be wiring. I'm pretty sure I got wiring so I start doing a whole scan of the area and start pulling things out of trees, sweeping through gutters, and we start finding bits of sections of a phone. Within a couple of hours we had enough portions of a phone to take it back to the lab and start doing searches and figure out the exact make and model of that phone. We got lucky. We had a part that had a very unique identifier number on it. It can take days if you don't have really good pieces.

"Looking back, there's a feeling of pride and satisfaction. That's one of the things that drives you to do this job, quite frankly. It's hard to go through a scene like that and not feel the pain of the people involved in it."

"What really sticks in your head is not what you see. It's the oppressive heat and the smell of burnt diesel and decay of bodies."

This story appears in the April 2017 Popular Mechanics.