Electrical Forensics In Action: The Boston Marathon Bombing

Marathon_BombingThe application of technical science as an enabler for the delivery of justice is a beautiful little niche of engineering.  The satisfaction that accompanies such work is one reason I was initially attracted to forensic engineering as a profession.  And oh what an honor it would be to work on the forensics team pouring over every little shard of metal and wire at the horrific scene in Boston.  Let’s hope the puzzle is completed before the cowardly perpetrator(s) get out of the country.

I was intrigued by be a high resolution video clip I saw on CNBC of the battery and wiring that was apparently used to detonate the device. The battery is a sub C-3000 nickel metal hydride cell by Tenergy.  Huh?  One has to ask why this battery was chosen by the individual who constructed this device instead of the far more common and easily obtainable options. First, we don’t know how many of the batteries were strung together, so I’ll avoid conclusions about voltage alone.  But the NiMH battery is a very high energy density, high current, low internal resistance battery with a long cycle lifetime.  It’s also rechargeable.  Why any of these features were deemed necessary for a one time use in a destructive device with very low current requirements is puzzling. Further, NiMH’s tend to self discharge very rapidly – 10-25% in the first day in some cases. This suggests the need to have charged them very recently prior to use.  A lighter, smaller, cheaper, longer shelf life, easier to obtain, less traceable solution might have been…. the ubiquitous square 9 volt that goes in your smoke alarm.

Tenergy NiMH battery used in one of the Boston marathon bombs.

Tenergy NiMH battery used in one of the Boston marathon bombs.

Also somewhat curious is the wire choice.  Did you see those wires?  Was he planning to jump start a car with that thing? They are enormous.  I didn’t get to my screen capture button quickly enough to have saved a frame with the markings on it, but those looked like 12 or 14 AWG aluminum to me.  That is some serious oversizing. Those sizes are good for a momentary 20 amps. A simple low-explosive detonation device might require about 1/100th of that, and even if more, only for a matter of milliseconds. Aside from detonation circuitry, let’s say, another 100mA for a timer or whatever control electronics were used. So presuming that one of the designer’s goals was to construct the device to be small, lightweight, and concealable, why the gross oversizing of both wires and batteries?

All this may tell us nothing more about the guy than that he doesn’t do much electrical design. Or, maybe that’s just what the script in the terrorist cookbook said to use. Even so, if your goal in life is to carry out this dreadful act, wouldn’t you have done some research and optimized your equipment? But that’s ok, cluelessness builds a criminal profile just as competence does, and the unique attributes of the components used may lead us directly to the source. Wires have their manufacturing date and origin, among other information, printed on them. We electrical forensics engineers rely heavily on these markings for cases ranging from building code compliance to electrical fires. Those wires may have been used out of laziness alone because they were part of a preassembled battery pack. Either way, the manufacturer will figure out when they made, and where they sent those finished goods, and a location & time of purchase will fall out of the investigation.

Still more information will come from the  internals of the battery – its chemical state of charge will tell a story. Engineers will measure the open circuit voltage, then subject it to a test discharge and plot the resulting voltage / current curves. These test curves start from the end point of the discharge curve that occurred on the actual day of use – ie while the device was energized, waiting to explode. Further, we know the exact point in time when the actual use curves terminated, if we accept that the battery was open circuited at time of detonation. The test discharge curves will be fitted to manufacturers data after accounting for age and estimated cycling history. Finally, the chemical forensics guys will be turned loose to cut open the cell and confirm or modify lifecycle assumptions based upon electrolyte condition. The state of charge will help to piece together what happened in the hours and days before the incident. For instance, If the battery was fully charged an hour before bomb placement…. hmmm, lets look at hotels & coffee shops in a close radius, etc. If the battery internals show signs of advanced cycling age, this may be an indication that the bomber didn’t buy it at all, but instead chopped the battery out of some existing equipment. In which case finding an RC car (that’s where these batt’s are often used) with a missing battery in a 1 room apartment somewhere will be a nice piece of evidence to tag. It’s only a matter of time.

I had a family member running in the marathon, and fortunately she was uninjured. My sincere condolences go to all those and their families who were less fortunate. Now in the wake of this latest tragedy  the race to deliver justice is on. There’s no question that the forensic teams working tirelessly around the clock right now will win. The more disturbing question, that I’ll leave for the behavioral psychologists to opine on, is how to prevent future occurrences. When someone wants to inflict terror to support their extremist ideology, all they need is an event, a crowd, a cookbook, and a willingness to die. The first 3 aren’t hard to come by and history has made clear that martyrdom runs deep in the terrorist circles. I don’t know the answer, but I suspect it will involve all of us sacrificing some (more) civil liberties. Just assume your bag will be searched, the NSA is listening to your cell phone call, and your ISP is in bed with law enforcement. For most of us, it doesn’t really make much of a difference, and alternatives that don’t fully exploit IT intelligence are certain to be much less palatable.

Jim Fink, P.E. – Electrical Forensic Engineer

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