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.

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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.

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