Hidden in some warehouse or basement, a terrorist builds a bomb.
As he unwraps a few pounds of C-4 explosive, the bombmaker's hands pick up bits of the distinct chemicals that make the bomb explode.
Those same particles are one way for investigators to later find the bomb before it can be detonated.
As an employee of the U.S. Department of Homeland Security, Michael Shepard is a chemist who works with developing technologies, including lasers, to find those specks, detect bombs and possibly prevent catastrophe. He spoke Monday night at the University of Tennessee at Chattanooga to a group of about 30 students and faculty about finding bombs before they go off.
Part of the problem with detecting bomb-making materials, he said, is that the bomb-maker not only touches the chemicals, he also touches his clothes, his sandwich, a soda can. A bit of mayonnaise, bread crumbs, sticky sugar from the soda can, oil from his skin can dilute the chemicals.
By the time he touches the door handle of the closet where he hides the bomb, maybe a few microscopic specks, 1 million times smaller than a grain of salt, are all that remain.
Terrance Brown, an 18-year-old UTC freshman, studies chemistry and didn't know that jobs such as Shepard's existed until he got an email about the lecture.
"I'm still trying to decide what I want to do with (the degree)," he said.
UTC associate professor Gretchen Potts invited Shepard as part of American Chemistry Society's Chattanooga Chapter monthly meeting.
Potts and Shepard studied together in graduate school at the University of Florida. Potts brings people who work in different areas of chemistry to show students their options after graduation.
Most of the technology Shepard manages is through programs tasked with developing equipment for Homeland Security-related agencies such as the Transportation Security Administration and the U.S. Secret Service.
Many of the devices under development scan moving groups of people to spot either chemical signatures or use X-rays to see if there are items hidden beneath clothing.
During his presentation, Shepard flipped through slides and photographs of exploded vehicles alongside a map of Mexico splayed on the wall.
"We always talk about the threat being 'over there,'" he said. "No, it's just south of the border."
And inside U.S. borders, also.
Shepard pointed to a basic diagram of a truck and a building -- pictures of the April 19, 1995, bombing of the Murrah Building in Oklahoma City.
"This is a very simple device," he said. The entire operation, from materials to convicted bomber Timothy McVeigh's food receipts, cost about $4,000.
Tim Pridemore, UTC emergency management specialist, asked Shepard if his technology would find a bomb in an area after it had been planted.
After a pause, Shepard said the only technology working in that way are bomb-sniffing dogs.
"My basset hound's better than your laser," Pridemore said to mild audience laughter.
"The problem is, after three hours your basset hound takes a nap," Shepard replied to more laughter.