Boston — Massachusetts Gov. Deval Patrick said this week that the two improvised explosive devices that exploded Monday near the finish line of the Boston Marathon were the only explosives found in the area. This contradicted earlier reports that police had carried out controlled detonations of other unexploded devices. How do bomb technicians carry out controlled detonations?
With an explosive device of their own. Such controlled detonations are rare. In most cases, bomb technicians aim to disrupt an explosive device — i.e., to break it apart so that it won't explode. Disruption is preferable to detonation for obvious reasons: It's less destructive, less dangerous, and more likely to yield useful evidence, since the shattered pieces of the neutralized bomb can be sent to a forensics lab for analysis. (An exploded bomb can still be analyzed, but it's a lot more complicated.) However, in unusual cases when improvised explosive devices cannot be disrupted and bomb technicians decide the best thing to do is to explode it, the technicians do what's known as a countercharge: They place another explosive next to the IED and intentionally detonate it to destroy the IED. For countercharges, bomb technicians employ a variety of explosive types and strengths, depending on the type of IED and the context.
For the most part, "render safe procedures" — the term used to describe bomb technicians' efforts to neutralize IEDs — are carried out remotely, via robot. Robots are often equipped with cameras or even X-ray capabilities that allow bomb technicians to examine suspicious packages from afar. Robots can also carry disrupters, which may be precision cutting tools, tools that resemble shotguns or small cannons, or other devices. And should a technician decide to countercharge an IED, the robot can carry that explosive to the IED. Contra cinematic representations, bomb technicians almost never attempt to dismantle an IED manually, a technique known as "hand entry," because doing so is extremely dangerous.