What Clues Do Explosions Leave Behind?
Incredibly useful ones.
By Forrest Wickman|Posted Tuesday, April 16, 2013, at 7:06 PM
In the aftermath of yesterday's bombing at the Boston Marathon,
investigators are working a 12-block crime scene to determine what kind of
bomb was used and what kind of bomber-domestic or international-planned the
attack. What sort of forensic evidence do explosions leave behind?
Bomb fragments, residue from the explosive, bits of electronics, and
sometimes even the bombers' own fingerprints and DNA. One of the first
things investigators do at a bombing site is determine what kind of
explosive was used. There are a few ways to do this. For example, forensic
scientists can use a handheld ion mobility spectrometer to sniff the air
around the blast for traces of the explosives. (This is similar to what's
used by the Transportation Security Administration if they ask you to step
into a puffer machine.) Some of these devices can identify the specific type
of explosive in the air, which is named in a digital readout. Other
investigators may run color tests, in which bomb residues are applied with
solutions containing reagents that change color in response to specific
explosives. Both of these tests are considered presumptive and can be
confirmed with procedures like chromatography and mass spectrometry back at
the lab.
If the explosive is unusual, investigators may be able to determine where it
was purchased. For example, some explosives are tagged with chemical
markers, called taggants, that identify where they came from. If the
explosive, on the other hand, is TNT, you know it was probably stolen, and
you can look over the mandatory theft reports filed to the Bureau of
Alcohol, Tobacco, Firearms and Explosives, which requires photo ID for all
purchases. (These thefts are reported many times a year, but investigators
may be able to incorporate evidence connected with the thefts into the
bombing investigation.) Unfortunately, if the explosive is homemade from
common ingredients, such as fertilizer or gun powder, then the chemicals are
difficult to trace.
Investigators also collect bomb fragments from throughout the blast area-and
in horrible cases like the Boston bombings, from the victims-in order to try
to reassemble the bomb. Once they have an idea of what the device looked
like, they can cross-reference it with the FBI's Terrorist Explosive Device
Analytical Center database, which collects all the improvised explosive
devices used by terrorists abroad. Individual groups tend to have their own
signature ways of making IEDs, using similar materials and designs, and the
FBI may be able to match the device with a pre-existing series of devices in
its database.
More-powerful bombs with softer casings tend to leave smaller fragments,
while bombs with harder casings, such as pipe bombs made of steel, can leave
fragments several inches long. If just the cap of the pipe bomb flies off,
investigators may find the whole pipe. However, even small fragments on
their own can be useful to investigators. For example, in the case of Pan Am
Flight 103, a fragment of a circuit board that investigators say they found
in a scorched shirt proved to be a crucial piece of evidence. Investigators
may also try to determine the origins of individual components, such as by
determining the model of watch used as a timer, or even the specific brand
of glue or electrical tape used to hold them bomb together.
If investigators are able to collect large enough fragments, they are
sometimes able to collect residual fingerprints or DNA evidence. One way to
detect and preserve fingerprints is through "superglue fuming": In this
technique, you heat superglue in a closed container with the piece of
evidence, and the superglue gathers around the fingerprint, highlighting and
preserving it. This is because superglue bonds remarkably well to the sweat
and residue on human fingers (which is also why it's easy to get your
fingers stuck together when handling superglue). On IEDs, however,
fingerprints are usually burned off any fragments, so investigators will
look for DNA. This may include "touch DNA": just touching something can
leave dead skin cells behind, especially if the surface is rough.
While these are most of the principal techniques investigators use to gather
clues at the crime scene, they're not the only ones. For example, if the
bomber leaves a bag or backpack, an investigator might inspect it for trace
elements such as pollen or pet hair. Pollen can be traced back to specific
types of flowers, and forensic botanists can cross-reference these flowers
against each other to determine where the bomber might have been. Pet hair
can be used to determine, for example, whether the bomber owns a specific
breed of dog. Once a suspect is identified, all of this evidence can be used
to search his or her home for matches.
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