Monday, April 22, 2013

Stories in the Smoke: What a Bomb Expert Sees

 

http://www.newyorker.com/online/blogs/elements/2013/04/stories-in-the-smoke-what-a-bomb-expert-sees.html?printable=true&currentPage=all

 

April 17, 2013

Stories in the Smoke: What a Bomb Expert Sees

Posted by Paige Williams

Dr. Adam B. Hall, a forensic chemist and former crime-scene analyst for the Massachusetts State Police, teaches at the Boston University School of Medicine, home to a highly regarded forensic-science program. He had just left campus on Monday, around 3 P.M., when ambulances screamed past him on Massachusetts Avenue, which connects the South End of the city with the Back Bay, where thousands of marathon runners were still making their way toward the finish line, at Copley Square. Hall figured the urgency was due to an unusually high number of fatigued or injured runners. Then along came a bomb-squad truck, an unmistakable sight for Hall, who has processed hundreds of crime scenes, most involving arson and explosives.

"People hear the word 'forensics' and they think autopsies, but there are many different areas in forensics," he told me Tuesday afternoon. "Forensic chemistry encompasses drugs, arson, and explosives, where you have chemical evidence; with shootings, stabbings, and sexual assaults, it's usually biological evidence."

By the time we spoke, investigators were engrossed in learning about the devices that, nearly twenty-four hours earlier, had instantly killed two marathon spectators, mortally wounded a third, and left more than a hundred and seventy others recovering or fighting for their lives. What were these devices? How were they made? What did they contain—carpentry nails, ball bearings, other projectiles? Details about a shredded backpack, six-liter pressure cookers, timers, and a circuit board were coming into play. Hall, though not working the marathon case himself, knew from experience that investigators would still be bagging the thousands of pieces of physical evidence at the detonation points and in the surrounding areas.

"I can imagine they're treating this almost like a plane crash," he said, "where they're collecting every single thing they possibly can because they don't know what is important and what isn't."

We met in the South End, where he teaches and where I live, less than a mile from the cordoned-off finish line. Hall had not yet been to the scene, so we walked over there together, in the sunshine. Ordinarily, Boylston and Newbury Streets are clogged with pedestrians, so much so that it's important for drivers to slow down even during green lights; yet beyond the repositioned metal barricades nothing moved but uniformed National Guardsmen and windblown trash. On the enlivened side of the perimeter, people went on with their week. They jogged, and walked to lunch, and took photos of Trinity Church, where in the foreground an American flag flew at half-staff. At Dartmouth and Stuart, Starbucks had set up an outdoor coffee station. The tripods of TV news crews forested the street corners and sidewalks. A woman walked a giant black poodle off leash, something no Back Bay Bostonian would attempt on a typical weekday, and stopped, on a bench, to watch the activity.

"I thought it was going to rain today," Hall said, looking up. Cloud cover would descend later, a chilling blanket of white, but for now there was clear blue. He wasn't just making conversation. "That's a consideration when you have an outdoor scene: How quickly do we have to do this? Evidence is transient."

Hall suggested authorities would be keen to learn how the devices were "initiated." In explosives investigations, "detonated" is not always the right term. "Low" explosives—such as black powder or smokeless powder, often used in pipe bombs—deflagrate. "High" explosives, known as ANFO (ammonium nitrate, or fertilizer, plus fuel oil) and T.A.T.P. (triacetone triperoxide, or a mixture of commercially available acetone, hydrogen peroxide, and something acidic, like vinegar) detonate. ("Oklahoma City was ANFO," Hall explained. "McVeigh and Nichols used a huge amount of ANFO in the back of a Ryder truck.") The distinction "relates more to the physics of the event," he said. "Is it a subsonic or supersonic explosion? Is it greater than or less than the speed of sound? Let's say you're at a fireworks demonstration and you see the explosion and then hear the boom. That means the speed of light is faster than the speed of sound."

Hall said he was particularly intrigued, looking at pictures from the marathon scene, by the smoke. It was white, and immediate, and there was lots of it, which he said isn't usually the case. "I was puzzled about that," he said. Hall is thirty-five but has silvering hair, buzzed short— not unlike that of military guys or the law-enforcement officers he worked with early in his career—and now he ran a hand through it. To him, the color of the smoke, and the volume, suggested a high explosive.

Investigators would be asking witnesses not only what they saw and smelled (you don't need to be a scientist to know that sulfur smells different than gasoline) but also what they heard. They would be listening to the blasts themselves, via valuable video footage—law-enforcement authorities have put out a call for any and all images. The trained ear can evaluate the brisance, or sound of an explosion, the way an oenophile might be able to tell a Chardonnay from a Pinot Grigio from a Sauvignon Blanc, or a gun enthusiast might tell a cap gun from a handgun from a shotgun. "Higher explosions tend to give a very loud crack rather than a low-toned boom," Hall said.

The investigators would also consider detonation velocity, the speed at which an explosive event occurs. "The energy of an explosion takes the path of least resistance, which is usually up and out," he said. "So if the device was on the ground or even in a trash can, it's hard for that energy to penetrate down into the ground. It's going to go up and out. If there's shrapnel added to it—that could be nails, bolts, razor blades, metal objects—all of those will go up and out, away from the device, traveling at the same rate of speed as the detonation." Both ANFO and T.A.T.P. explosions have detonation velocities in excess of fifteen thousand feet per second, he said, meaning that one second after such a device activates, its pieces might be found fifteen thousand feet away. "That's why they had to set the perimeter so large"—by now the original fifteen blocks had been scaled down to twelve—"and have this huge disruption to the city. There could be pieces of shrapnel lodged in the side of a building hundreds of yards away."

As we walked, wind whipping the coattails of passersby, Hall occasionally dropped his voice, to avoid being overheard. "When I teach, I tell students not to talk about their work in public," he said, as we looked for a place to get in out of the bluster. "It freaks people out. Plus, at a time like this, when everybody's on heightened alert…"

We went into Clery's, an Irish pub, to have a cup of coffee. A marathon runner was sitting at the bar in his neon race jacket, scrolling through items on his iPhone.

The method of detonation, Hall went on, would likely reveal clues to the attacker's level of sophistication and whether he or she worked alone. The device had to be small enough to carry, and put together in such a way that left the assailant enough time to flee. Tuesday night, there were reports that the bombs had been set with a simple timer. If they hadn't—and Hall confessed that this thought had frightened him—the devices might have been activated from a distance by cell phone or some other electronic means. "If they were initiated remotely, I don't believe that's the type of individual who just had a bad day or was mentally unstable and said, 'If I can't be famous I want to be infamous,' " he said. "If it was remotely initiated, it may not suggest a huge organization, but it does suggest this was more organized than if somebody was just messing around."

Remote activations and extra shrapnel are the kinds of tactics explosives experts usually see overseas, he said. Iraq, Afghanistan. "We don't usually see that domestically," he said.

"They'll put emphasis on things like victims' clothing. They'll be looking for things that look out of place—electronic components or burnt wires." He used poured sugar and tiny creamer containers and a spoon smacked against a saucer to make a point about friction, and about the energy levels required to detonate certain explosions. He didn't want to get into too many specifics about how these things might be done. "The problem with this is, there were so many people in the area, there's trash, water bottles, newspapers blowing all over the place, so if you find electronic components, is it the cell phone of a victim or is that something that was part of the original device?"

These are the kinds of questions he poses to students in the Analysis of Ignitable Liquids and Explosives and Special Topics in Analytical Chemistry classes he teaches, and in the two labs he runs at Boston University. Off-site, Hall creates mock evidence to be brought back to campus for analysis. His students go on to work in police departments, firearms units, crime labs, medical examiners' offices. He is fond of analogies, and makes good ones.

I asked him what the twelve seconds between blasts suggested to him. "It may tell you there's more than one person involved, or that the explosions were initiated remotely," he said. "Because of the timing and distance, it doesn't seem possible that one person could initiate the device and get to the second location, considering the crowd and how congested it was." Two actors or remote detonation also might explain why the first blast occurred near the finish line and the second further back on the race route.

On my iPhone, we looked at raw Boston.com footage showing the explosion closest to the finish line. "Eight-point-six million views," he murmured, and then put the speaker directly to his ear. The loud crack of the first blast was audible, and then came screaming. After listening twice, he watched it. "The thing that leads you to think more toward high explosive is that if you look at the very center of it, it's very bright. It happens extremely quickly—you see that fireball in the center and then the huge amount of smoke."

The captured moment of the blast is invaluable, he said, though the full picture won't become clear until the scientists start working with all the evidence in the lab.

As we left Clery's, turning once again onto Dartmouth Street, I wondered about why he left law enforcement. Hall quit the state police six years ago, to teach at B.U. and to earn his Ph.D. in analytical chemistry at Northeastern. He had worked too many consecutive cases involving the deaths of children, he said. Now, though, a big investigation unfolding, he was feeling "antsy" for a call returning him to the field. As we made our way once again to the South End, the crime scene at our back, he looked over his shoulder and said, "I just really want to be there."

Photograph, of smoke in the street after the Boston Marathon explosions, by David L. Ryan/The Boston Globe/Getty.

 

 

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