Improvised explosive devices (IEDs) blew chaos and fear into residential neighborhoods in New York City and New Jersey, where a blast hit festivities at a five-kilometer charity race to benefit U.S. Marines and sailors, over the past weekend. The incidents follow a familiar pattern in which homemade devices made from pressure cookers and pipes are loaded with ball bearings and other metal objects, and detonated using chemicals that can easily be obtained at retail stores or online.

The first bomb rocked Seaside Park, N.J., on Saturday morning as the Seaside Semper Five race was about to start. Several “pipe bomb-type devices” had been wired together and placed in a plastic garbage can, but the explosion did not cause any injuries. But at 8:30 p.m. a bomb placed in or on a dumpster injured 29 people in New York City’s busy Chelsea neighborhood, where the blast damaged surrounding buildings and shattered nearby windows. Soon after that police discovered a second explosive device made using a pressure cooker a few blocks away. A mobile phone had been taped to the device, apparently as part of a remote detonation mechanism.

Still another incident rattled nerves Sunday evening, when two men found a backpack stuffed with as many as five IEDs near a train station in Elizabeth, N.J. One of the bombs detonated as police tried to disarm it with a robot. Law enforcement officials on Monday apprehended Ahmad Khan Rahami, a 28-year-old naturalized U.S. citizen of Afghan descent, in connection with the bombings.

A federal law enforcement official told the Associated Press that the Chelsea explosion left a residue of Tannerite—the brand name of a substance made up of ammonium nitrate and aluminum powder and designed to explode only when the two chemicals are combined and hit by a high-velocity bullet from a firearm. Tannerite, primarily used at firing ranges to create exploding targets, needs a relatively high-energy impact to set it off, and cannot be directly detonated with a simple flaming fuse or electrical signal from a cell phone alone.

New York Times correspondent Rukmini Callimachi reported Monday on Twitter that the second Chelsea bomb and at least one of the devices in Elizabeth contained hexamethylene triperoxide diamine (HMTD), an explosive organic compound. HMTD is similar to triacetone triperoxide (TATP), which was used in the 2005 London subway bombings, the December 2001 “shoe bomber” attempt to blow up an airliner and the train station and airport bombings in Brussels earlier this year.

Tannerite is something that must be purchased whereas HMTD is something you have to make—two very different approaches to making bombs, says Jimmie Oxley, co-director of the University of Rhode Island’s Center of Excellence in Explosives Detection, Mitigation and Response. Illicit bomb makers have to decide which explosive material they want to use based on the materials available to them, then they choose a container—such as a pressure cooker or pipe—to carry that “filler”, Oxley says. “It would be unusual if the same bomb maker used Tannerite and HMTD as fillers in different bombs,” she says. It is more likely, if those compounds were in fact used in these most recent bombings, that the HMTD might have been used as a detonator to initiate the Tannerite, she adds.

The Semper Five bombing, in particular, is eerily similar to the explosion near the finish line of the 117th Boston Marathon that killed three people and injured 264 on April 15, 2013. That explosion set off a manhunt for Chechen brothers Tamerlan Tsarnaev, who died in a shootout with police, and Dzhokhar Tsarnaev, now on death row in a Colorado prison. Soon after that bombing Scientific American spoke with A. J. Clark, a former military intelligence analyst and president of Thermopylae Sciences + Technology, a provider of geospatial intelligence technology. In that 2013 interview, portions of which appear below, Clark provided insights into how IEDs work and how terrorists use them to launch attacks.

[An edited transcript of the interview follows.]

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 war zone 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 five-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.