The sophistication of the package bombs that keep on detonating throughout the cities of Austin and San Antonio in Texas has authorities and experts concerned that the perpetrator is not an amateur, USA Today reported this week. But engineer or not, they are leaving a growing trail of evidence like an undetonated bomb left at a FedEx location.
Per the Washington Post, the bombing spree began on March 2nd and started with explosives being left on people's doorsteps in east Austin; it has since involved a sophisticated tripwire-enabled bomb on a residential street and an attempt to mail explosives through a delivery company. In total, five explosions have been confirmed tied to the case, resulting in at least two deaths and five injuries. (Per CNN, another explosion at a Goodwill store in Austin on Tuesday is believed to be unconnected.)
Former FBI bomb technician Danny Defenbaugh told USA Today that the number of devices and their varying complexity likely has authorities looking for someone with a formal or professional background in engineering, and that serial bombing campaigns of this variety typically involves "looking beyond a person who simply searched the Internet for how to build these things."
"That fact that someone could build these devices, including the one with the tripwire mechanism, and not blow himself up, that means something," Defenbaugh added. "That's why they have hundreds of people working on this."
Yet the more bombs that go off, the more evidence law enforcement is able to find. As the New York Times reported, explosions "do not destroy evidence of the bombs' origins so much as blast it into many bits and pieces." Every bombing provides more clues, right down to shrapnel, which could narrow down the possible range of locations where the suspect purchased supplies. The composition of surviving parts like switches, relays, and homemade circuit boards, or analysis of residual chemical traces, could also shed light on where the bomber may have bought material - and other evidence like DNA could potentially survive as well.
The Times added:
"A bomber often believes that the device itself is consumed in the thermal effect of the blast, and that he is giving himself distance from the criminal act," said a federal agent and explosives expert who spoke on the condition of anonymity because he was not authorised to speak to the media. "They think they have autonomy, but they don't."
Investigators now are going so far as to reconstruct the bombs that exploded, using similar materials. "Each one of these devices is being rebuilt to help investigators figure out what they are looking for," the federal agent explained. "It is like putting a puzzle together."
Analysis at the FBI's lab in Quantico, Virginia, has led investigators to conclude all of the devices were built by the same person or group of persons.
The recovery of at least one unexploded device from a FedEx facility in San Antonio on Tuesday is significant because it will allow authorities to inspect one of the bomb-maker's creations without the need to forensically reconstruct it. According to the Associated Press, officials say that the bomber has been generally successful at evading surveillance, but that a security camera at the FedEx building may have captured a view of a suspect.
Yet Rand Corp analyst Brian Jenkins told NBC that in lieu of evidence like a manifesto or written threats, authorities often find that serial bombers have taken care to cover their tracks.
"This requires reconnaissance," Jenkins said. "This requires target selection. They have to think about building a device that works. They have to build that device. They have to think about delivering that device in a way that enables them to conceal their identity."