In light of the bombing at the Boston Marathon, I thought some background on explosives and the investigative techniques used to track down bombers would be helpful.
The main difference between an explosion and a simple fire is the rate at which they consume their fuel. A fire burns its fuel at a gradual rate, but a bomb uses up its fuel almost instantaneously. Finding the remnants of the bomb can be a challenge, since the device is blown apart over a wide area. However, a non-incendiary bomb often leaves behind evidence, simply because that evidence is blown away from the explosion. When terrorists attacked the World Trade Center with a van filled with high explosive in 1993, over 700 FBI agents combed through the rubble. They found a piece of a vehicle that included the Vehicle Identification Number (VIN); the way it was ripped apart was a sign it was the vehicle that delivered the explosive. The VIN was for a van that had been rented and then later reported stolen. When the renter, Mohammad Salameh, came to the rental agency to get his $400.00 deposit back, the FBI was waiting for him. His arrest led to the other co-conspirators in the case.
Bomb experts can tell a lot by the construction of the bomb: what explosive was used, how the device was triggered, how it was delivered. Lab technicians will dissolve explosive residue with a solvent such as acetone and then analyze it, likely using gas chromatography as well as mass and infrared spectroscopy. That will give investigators the particular explosive that was used, and they will track down sellers and buyers of that explosive. In Boston, the bomber used ball bearings as shrapnel to increase the bomb’s effectiveness. You can bet the FBI is looking into ball bearing purchases in the Boston area over the past year.
One relative blessing at the Marathon was the devices appear to have been constructed using low explosives rather than high explosives. Explosives are categorized by the speed of the concussive blast they create. As a point of reference, the charge in a cartridge shoots a bullet from a gun at around 1000 feet per second. A low explosive creates a concussive blast that travels in the 3000-7000 feet per second range. That’s still devastating, especially when it’s propelling ball bearings at that rate, but if it had been high explosive the damage would have been exponentially worse. High explosive has a concussive wave speed in the 20,000-25,000 feet per second range.
High explosives can be nitroglycerine-based, such as dynamite and TNT. These days they’ve mostly been replaced by what’s called ANFO, which stands for Ammonium Nitrate – Fuel Oil. Mixing those two items creates a stable and very powerful explosive. Unfortunately these items are easily available. In 1995, a Ryder truck filled with commercial fertilizer and diesel fuel blew up in front of the Alfred P. Murrah Federal Building in Oklahoma City. Once again a VIN was the key to the investigation. The FBI recovered the rear axle of the truck that included the VIN and traced it to a shop in Junction City, KS where it was rented. A false ID was used, but people at the shop helped a sketch artist produce a picture of the renter that was a remarkably accurate likeness of Timothy McVeigh.
In style, the Boston bombing has similarities to the Centennial Olympic Park bombing in Atlanta in 1996 in that a back pack was used and the target was a crowd attending a sporting event. Shrapnel was also what caused the deaths and injuries there. The bomber in that case, Eric Robert Rudolph, used high explosive in three pipe bombs that were surrounded by nails. Thanks to a vigilant security guard, Richard Jewell, the back pack was noticed and authorities had begun evacuating the area when the bomb detonated, or the death toll might have been higher.
There are two lessons from the Centennial Olympic Park incident. One, initial assumptions are likely wrong. In that case, Jewell was falsely identified by news organizations as a suspect in the bombing. It turned Jewell’s life into a nightmare, when he should have been hailed as a hero. Shortly after the Boston bombing there were broadcast reports that a Saudi national was being questioned, with the implication of him being an al Qaeda-style terrorist. Bombs, though, create their own smokescreens.
The second lesson is that the investigation will be implacable. After the Centennial Olympic Park bombing, the investigation came to a standstill for a year, until Rudolph exploded bombs at an abortion clinic and a lesbian nightclub in the Atlanta area. Later another clinic in the Birmingham, AL, area was bombed. The reconstructed bombs showed that the same bomber had done all four attacks, and a partial license plate number seen at the Birmingham bombing led the FBI to Rudolph. Rudolph evaded arrest and went on the run in the Appalachian area of the Carolinas. He was put on the 10 Most Wanted list and there was a million dollar reward on his head. It was over five years before he was caught, but the manhunt remained active and he was finally arrested in Murphy, North Carolina in May of 2003. He’s now serving four life terms without the possibility of parole at the Federal super-max prison in Colorado. That prison is also the home of other terrorist bombers.
They will have a cell ready there for whoever committed the Boston Marathon attack, and the police and federal authorities will pursue this case until the bomber is identified and captured.