Boston — In the aftermath of Monday'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.