The bombing near the finish line of the 117th Boston Marathon on Monday killed two and injured more than 100 people on site. Now comes the search for who planted and detonated the explosives, and the motive.

The first bomb was detonated at about 2:45 P.M. local time near one of the many classic storefronts lining the marathon’s home stretch. The second explosive followed within minutes about 50 to 100 yards away. Law enforcement later found and dismantled at least two more explosive devices, according to various news reports.

Improvised explosive devices (IEDs) such as those used to attack the marathon are sometimes triggered remotely by cell phones. The Associated Press initially reported that law enforcement had the cell network in the vicinity of the finish line shut down after the incident, but later reports contradicted this, indicating that problems receiving a signal were due to the volume of cell phone users on the network.

Federal authorities classified the bombings as a terrorist attack as of Monday evening and indicated that both of the detonated bombs were small and did not contain C-4 or other high-grade explosives, CNN reported. Explosive devices have their own signatures, even those that have been pulverized by the force of their detonation.

Scientific American spoke with AJ Clark, a former military intelligence analyst with deployments to Iraq and Guantanamo Bay, Cuba, about how IEDs work, how they are used and how they can be used to find those behind such an attack.

[An edited transcript of the interview follows.]

Reports indicate that IEDs were responsible Monday’s attack on bystanders at the Boston Marathon. Most people have heard the term “IED” in relation to combat in Afghanistan and Iraq, but what, exactly, is an IED?
Most improvised explosive devices include the following components: a cheap cell phone, electrical wire, a fuse, batteries (AA or 9-volt), electrical tape and a thyristor (a solid-state semiconductor device). This last piece provides the option of letting you wire into the positive and negative diodes of the speaker on the cell phone board.

Two common ways to complete the circuit between these components is to use the vibrating mechanism on the cell phone or the speaker—this could be either the speaker used to make your phone a speaker phone, or the speaker that you put up to your ear during a normal conversation. We found that the speaker phone has more power going to it and was more commonly used. When the phone is called, it activates the ringer, which makes the connection between those two components and kicks off the signal to detonate the explosive.

The typical makeup when you find these things will be based on the contents that the bomb maker had at their disposal and what it is they are trying to do. Over in Iraq, we had a lot of what are called explosively formed penetrators, where whole IED is designed to explode up under a vehicle to create the greatest destruction possible. In something that is meant to detonate in a crowd of people, a bomb maker would pack into the IED whatever they had at their disposal—such as nails or ball bearings.

How are IEDs different in a war zone, as opposed to something that is used in a city or other civilian area?
You wrap the fragments around some type of explosive or ordinance. In a warzone you have lots of access to ordinance. Here you probably would probably try to find an explosive for blasting, say dynamite or C-4. Generally speaking a person wouldn’t use fertilizer in this situation, the way they did during the Oklahoma City bombing, because you need a lot of fertilizer—enough to fill a 5-gallon bucket or larger drum—to generate a decent-sized explosion. In a situation like this, you’re going to look for a something denser that you could put in a small location and then attach your cellular trigger to it.

How are these explosives detonated remotely?
The concept is, I need to create the signal between my positive and negative circuit and then set off my fuse. The problem with a timed fuse is that it doesn’t necessarily happen when they want it to.

How do cell phones change this?
The distance someone can be from the blast area is much greater. Generally speaking someone is in a crowd when they use these. Cell phones are also more reliable than other methods of detonating explosives. Within about 30 minutes, someone can buy a $10 phone and be able to set off the device.

What can an examination of the detonated device tell investigators?
We can look and see the method of operations or the signature that the bomb maker had in terms of how they set up the fuse and what type of material they used. That’s our best way of tracking down who is responsible.

How badly are these bombs damaged during a detonation (such that it’s difficult to analyze the pieces for criminal evidence)?
It varies, but you’ll find remnants, wires for example. You might even find a scrap like a red piece of tape that looks the kind of tape used at another bomb site. They’ll also see the complexity of how the bomb was put together. That’s really important too. Some bomb makers will also use other pieces of technology, like using a thyristor instead of connecting directly to the battery. A thyristor acts as a switch when an electrical signal is sent to it. That’s directly connected to the positive and negative diodes in the cell phone. Where you’ll get a real break is if a second or third bomb is found before detonation and the bomb squad is able to dismantle them and analyze how they were built.

Early reports out of Boston were that law enforcement had the cell network in the area around the finish line shut down (although the mobile service providers disputed that they had shut down their networks). Regardless, why would law enforcement want to cut off cell service?
In shutting down cell service, you block the ability to create that circuit and ignite that fuse. In the military we would use jamming devices on our trucks, sometimes seven or eight of them on a single vehicle. Some of them work, some don’t. If law enforcement in Boston did shut down the service it could have been because they found other, undetonated IEDs that were equipped with cell phones.