As rumors flew Wednesday that a suspect had been identified in the Boston Marathon bombing, investigators continued their careful analysis of evidence. Adam B. Hall, an instructor in the biomedical forensic sciences program at Boston University School of Medicine who previously worked in the Massachusetts State Police crime laboratory, described how such investigations are likely to unfold and why they will be essential in coming days and weeks.
Reconstructing the specifics of the bomb and discerning its components may seem tangential to the primary goal of finding the perpetrator and healing the wounded. But such evidence can help law enforcement officials build their case against a suspect and may provide leads in identifying who, how, and why the perpetrator did it.
Suspects are often identified through more traditional methods of police work, including reviewing video footage, surveillance, and informants, said Hall, who is not involved in the investigation of Monday’s bombing.
“As suspects are generated, if there’s a search warrant that’s collected and executed on someone’s property or a vehicle and there are components similar to or consistent with those that were used to make one or more of the devices, they really help to provide some of the circumstantial evidence against a person,” he said.
For five years, Hall worked in the Massachusetts State Police crime laboratory, where he analyzed a variety of cases ranging from suspicious packages to the remains of mailboxes blown up by small bombs built by a neighborhood teenager. Because this is such a large event and involves local, state, and federal investigators, evidence collected from the crime scene will be sent to an FBI laboratory in Quantico, Va.
“Their job is essentially to do a detailed analysis . . . everything from fingerprints to what the device looked like, trying to reconstruct the device from all the pieces,” Hall said. “It becomes a massive jigsaw puzzle at that point.”
The evidence gathered could span a wide range, from pieces of the actual exploded bomb to evidence taken from the clothes or even bodies of people standing closest to the bombs, Hall said. Even though the bomb exploded, there is always some trace amount of material left over.
“Sometimes, it’s as simple as taking things that look like a Q-tip to a road sign that was adjacent to an explosive device that went off, that provides a surface that would collect some of those residues,” Hall said.
Chemical analysis of residues can reveal what the explosive is, and that provides clues about what types of evidence to look for during searches.
Hall said that black powder or smokeless powder, a form of gunpowder, is widely available in some states. Gun owners sometimes use smokeless powder to reuse ammunition, Hall said, and in New Hampshire, for example, it is possible to buy such materials with few restrictions. A form of gunpowder was used in the Atlanta Olympics pipe bombing in 1996.
Fertilizer has been used in other bombs, such as ones in Oklahoma City, which used ammonium nitrate, and in 1993 under the World Trade Center, which used urea nitrate.
Hall said noted that he had no inside information about the Marathon bombs, but said that his own viewing of the video of the explosions suggested to him that they were not caused by a “low explosive” like black powder. While he said visual and auditory information was far from enough to rule out or confirm any particular explosive, the large amount of white smoke and the sound of the explosion point to more powerful “high explosives.”
Knowing the components of the bomb may provide clues about how it was built. Unusual materials might help investigators understand whether it was made in the United States or abroad, for example. And reconstructing the bomb may provide hallmarks that tell investigators who made it or how it was designed.
“If they see any particular types of signatures, that would be characteristic of certain groups they’ve encountered,” Hall said. “The way I think of this is when you’re a child, one of your parents teaches you how to tie your shoes: the loop, swoop, and pull, versus two rabbit ears. The way someone ties a knot, or the way they twist wires together, or whether they choose to initiate a device with this type of a timer versus that type of a timer” could be telling.
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