Inside the FBI bomb squad

An exclusive look at how investigators are trained to spot clues in the aftermath of an explosion.
2:47 | 09/16/17

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Transcript for Inside the FBI bomb squad
Today marks one year since the crowded New York City neighborhood of Chelsea and downtown was rocked by a bombing. More than 30 hurt. When jury selection started for Ahmad Rahimi's trial, ABC news took part in the training exercise with the men who teach police the forensics, basically the CSI of investigating bombs and here's Pierre Thomas. Three, two, one. Reporter: The power of a bomb. Devastating. Often lethal and at an undisclosed location in New York elite bomb technicians trained police and federal agents in a craft being put to use all too frequent frequently. Simple explosives. Some made of commonly available chemicals. We're not talking about large amounts of material needed to do incredible damage. It's just shows what a little bit can do. Reporter: Friday's attack in London, the bombing at a concert in Manchester and last year's attack in New York's Chelsea neighborhood are painful reminders of the ongoing threat. As an FBI a few months into nye role I responded to the Chelsea bombing and our job to respond to the worst and this is the best train on what to look for after the bombing. The simplicity leaves that weapon in the hands of anyone would wants to cause harm to innocent people. Reporter: Despite the destructive force, bombs leave behind critical clues. This is what's left of a precious cooker used in the Boston marathon bombing. The misnomer is in an explosive crime scene things vap vaporize. They don't. It may not be in its original Tate but it's still there. Reporter: In this training exercise, authorities set off a series of car bombs. Go back to the tape. Rewind in slow motion. The investigative premise, if you hunt for the blown apart pieces at the bomb detonation E epicenter you can see essentially find the bomb and put it back together. A key to success knowing how and where to look. For example, new bomb forensic investigators are trained to look everywhere including up. In this exercise a trigger was placed in the trigger to see if the trainees could find it. Look there. Wires, electrical component, a cell phone. Inside the vehicle, there may be fingerprints or DNA to be collected. And if there's a timer or clock where was it sold? To whom? Even the screws used as shrapnel may lead to a store where there might be surveillance footage. Every blast site, a potential treasure trove of forensic evidence as they're hopefully finding in London. Dan, Paula. What a fascinating look. Thanks Pierre.

This transcript has been automatically generated and may not be 100% accurate.

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