A series of bombings in Austin, TX has residents on edge. Little is known by local Law Enforcement at this time and Federal resources are currently assisting with the investigation. More on the story can be found here:
My hunch is that these devices are being initiated using electricity, and a relatively low-tech type of firing switch known as a “pull-loop”. The main charge likely consists of either TATP (tri-acetone tri-peroxide) or, smokeless black powder. It’s also possible that some sort of arming switch is being used, allowing the bomber to emplace victim-operated devices, arm them, and escape without self-injury. The bomber’s current successful track record suggests a relatively high level of expertise, despite the relatively low level of technology employed.
During my time as a US Army Special Forces soldier, I was fortunate enough to gain some real world experience dealing with improvised explosive devices, commonly known as IEDs. I was also lucky enough to receive additional advanced training. The IEDs in this case could also be described more commonly as “booby-traps”, given 100% of the devices so far are “victim operated”. Victim operation alludes to the requirement for the victim to initiate some sort of movement/disturbance in order to cause the device to explode. To get a bit more clarity on exactly what IEDs are, let’s break things down into the basics of construction.
The components of any IED can be described using the acronym “PIMFAC”. PIMFAC stands for: Power, Initiator, Main Charge, Firing Switch, Arming Switch, and Container.
It should be noted that much detail has been left out due to the sensitive nature of this topic. Let’s get started.
P: Power Source
A power source is not always required. However, given the broad potential for creativity when electrically fired devices are used, a typical IED uses electricity to initiate detonation. Chemical reaction may be used in place of electricity. And, applying a fuse to open flame or, sufficient shock, can also be used to achieve detonation. The “Christmas Day Bombing” in 2009 would be an example of a “non-electric” device used in a real world application. The initiator used in this case was chemical in nature and did not require electricity. This also assisted the bomber when passing through metal detectors, remaining undetected.
When electricity is used, a very common source is from a standard 9 volt battery.
Think of the initiator as the device that applies enough heat or shock to the actual main explosive charge to “initiate” detonation. One example would be a commercially manufactured electric blasting cap. When exposed to ample current, this type of initiator essentially converts electricity into heat. This heat is sufficient enough to initiate a small explosive chain inside the blasting cap. The end result is a miniature “shock front” or, detonation. Ideally, this is enough to cause the main explosive charge to detonate.
Devices used during the Boston Bombings utilized small light bulbs as part of the initiation process. The bulbs supplied the necessary heat required to cause the contained propellant to burn, rather than “explode”.
M: Main Charge
The main charge is the body of the main explosive charge itself. The compound used can be manufactured in a laboratory or even manufactured as a “homemade explosive” (HME). HME can be extremely unstable and it should be noted that the instability can in crease over time. The detonation of the main charge creates the velocities needed to create lethal projectiles and to cause damage to soft tissue near the epicenter of the explosion. In the 1993 world trade center bombings, the main charge consisted of Urea Nitrate augmented by canisters of hydrogen, aluminum, and magnesium. The main charge was detonated via a smaller “booster” consisting in part of nitroglycerine dynamite.
A main charge can also consist of a propellant instead of an explosive. A propellant burns instead of detonating. Gasoline is a propellant, as is gun powder or “smokeless black powder”. The devices used during the Boston Marathon Bombing of 2013 consisted of smoke less black powder contained inside of pressure cookers. The pressure cookers provided the container necessary to allow the expansion of the burning propellant to reach the critical state of detonation, creating a high velocity a shock front.
F: Firing Switch
The firing switch is an incredibly interesting component when analyzing potential applications, targets, etc. For example, from the perspective of an investigator, this component will shed light on precisely how a device could be employed. This can allude to potential targets and target locations.
The firing switch used in the initiation of the Boston bombing devices was a simple kitchen timer. If a timer were located at a potential bomb making facility, this would give vital information to the investigator. We know that the devices used in the Austin bombings are not utilizing timers as the means of “firing”. A timer would detonate at a specific time or, when time has expired, making this type of device more likely to be used indiscriminately for example, at a large gathering, as opposed to a situation where a single victim would be harmed as a result of some sort of physical input like picking up or shaking.
A: Arming Switch
An arming switch can be vital for the individual emplacing the IED. The IED will need to be transported safely, but still relatively intact to allow the individual emplacing the IED to minimize time on the objective, thereby limiting exposure to potential capture/compromise. The arming switch can be thought of like a safeguard. It ensures no electricity can flow to the initiator until the device is “armed”.
Imagine a light switch. The light switch could be thought of as the firing switch and the light bulb could even be thought of as the initiator. In this example, if the light switch were “flipped” no power would flow to the light bulb until the device were “armed”. One could conceptualize the arming switch as an additional light switch. So, the light bulb would require two switches to be thrown prior to initiation. Arming switches are sometimes accompanied by some sort of “safe to arm” indicator. A simple LED is sometimes used. If the LED were powered “on” this could indicate that power is flowing through the firing switch as well. If the device were armed in this state, the device will prematurely detonate.
Arming switches can be far more complicated and even armed on command, after the bomber has emplaced the IED and left the area.
Given the fact that the devices used in the Austin bombings are in fact victim operated, and victim operated devices can be unstable (requiring small physical input from the potential victim to set off detonation) it’s highly likely that arming switches are being employed.
A container can be as simple as a pipe. “Pipe bombs” typically use a basic propellant such as smokeless black powder and the container itself fragments into projectiles during explosion. A container isn’t always necessary. But homemade explosives will often be accompanied with a container due to their consistency. The 1993 Oklahoma City bombing used Urea Nitrate as the main charge. The Urea Nitrate utilized barrels as containers. These barrels were meant to house the urea nitrate as opposed to fragment into projectiles during detonation. The container used in the Central Park detonation in July of 2016 utilized a paper bag. This explosive also utilized TATP as it’s main charge.
Other objects meant to function as lethal projectiles can also accompany containers. Nails, ball bearings, etc. have all been used in previous attacks.
The Austin Improvised Explosive Devices
With some basic knowledge of the components that make up an IED, we can take the information we have about the Austin bombings and begin to reverse engineer the types of devices likely being used.
P: Power Source
Recent events have primarily utilized electricity to initiate an explosive chain. It is highly likely Austin devices are also using electric current. These types of devices allow the IED maker tremendous breadth in selecting applications for victim operation.
In this case, the power source is likely a common 9V battery or series of batteries.
The initiator in this case is likely improvised. A common bulb accompanied by another compound that is highly sensitive to heat/flame could be an effective means. This would be similar to devices used in Boston. This method could be applied to a main charge consisting of a propellant and even to certain types of explosives.
M: Main Charge
The main charge is possibly made up of a common propellant such as smokeless black powder. But, this would need to be accompanied by a container. If there was a lack of container fragments found at the crime scene, this could rule out the use of propellant as the main charge, since a propellant requires a robust container such as a pipe or a pressure cooker to be effective. Another likely option for the main charge is TATP. TATP is fairly simple to create but highly unstable. However, it is also very sensitive to heat, allowing for the use of a simple electric initiator.
Given the seemingly immediate and reliable detonation of all four devices thus far, my guess is TATP. A fifth device recently exploded in a Fed-Ex shipping center while in motion on a conveyor belt. Authorities have not confirmed that this device is linked to the four bombings in Austin. However, if so, we now have a stronger sense of the type of main charge being used. The main charge is more likely a fairly sensitive homemade explosive as opposed to a propellant. However, this could also suggest a very sensitive firing switch that failed in transit, as opposed to a highly sensitive homemade explosive that detonated because of shock etc.
It should also be noted that recent directives from open source extremist websites suggest using “pressure cooker bombs” (using propellants) on large crowds, such as during the Boston bombings. The events in Austin are more along the lines of precision applications, geared towards a single victim. This further supports the idea of an explosive over a propellant as the main charge.
F: Firing Switch
We know so far that all of the devices employed have been “victim operated”. This suggests a myriad of possibilities while ruling out others.
As mentioned previously, the method of firing can give tremendous insight when conducting overall analysis. A common victim operated firing switch is known as a “pull-loop”. It’s very likely a pull loop has been employed in all of the package bombs in Austin thus far. Simple yet effective, a pull loop switch closes a circuit, allowing power to flow to the initiator.
The fourth bombing utilized a tripwire. While the type of firing switch is likely different from the types of switches employed previously, the core of the firing mechanisms are likely very similar and still rudimentary.
A: Arming Switch
An arming switch may or may not be employed by the bomber. However, an arming switch is likely given the bomber has successfully employed four (possibly five) devices with no premature detonation. If an arming switch is being used, it may be as simple as a toggle switch.
The containers in these attacks have in part been used to mask the device inside. It is not yet known if fragments of containers have been found on scene. However, projectiles meant to maim/kill victims have been used.
In summary, the devices used thus far in the Texas bombings are likely simple but have proven to be effective. They are likely battery powered, utilizing rudimentary firing switches, an arming switch, and a sensitive homemade explosive such as TATP.