At least five Dallas police officers were killed and nine wounded July 7, after a peaceful protest over recent police shootings. Here's what we know so far. (Deirdra O'Regan/The Washington Post)

Reports that Dallas police used an explosive-armed robot to incapacitate the man suspected of killing five officers Thursday night prompted a flurry of articles about the novelty of the technique and its repercussions. (It appears to have been the first incident of its kind, for what it's worth; and the repercussions are to be determined.) What's less clear is how exactly the process worked.

We know where the final confrontation took place. Video from the incident shows a suspect firing from outside Building A at El Centro College. Across the street is a brick building that, on its south side, includes a parking garage. It was there that the suspect was cornered.


We know, too, that the Dallas police department used a robotic device to approach the suspect. It's not clear exactly what sort of robot it was. When a man shot at Dallas police headquarters last year and was tracked to Hutchins, Tex., a robot with an articulated arm was used to search his vehicle for explosives (after a sharpshooter had shot him from some distance). You can see it at work here, though it's not clear whether this is Dallas's robot or Hutchins's.

The New York Times explains how those robots work in general, if you're curious. More interesting to us was what the Dallas robot was carrying. After all, we know police have robots that can manipulate the environment. What may be surprising to people is that they also have access to explosives powerful enough to kill a person.

Matt Barnett runs Bonetti Explosives in Columbus, Tex., and he explained by phone both what sorts of explosives a police department would have on hand and how those devices might end up killing a suspect, as they did early Friday morning.

We should start with the latter question. Explosives kill in two ways, through fragmentation and overpressure. Fragmentation occurs when an explosive is encased in something that is blown into fragments — fragments that are then propelled outward in every direction like little misshapen bullets.

Overpressure results from the explosion's shock wave, which creates pressure that can cause internal bleeding or can otherwise damage organs.

The best description of the differences here comes from an experiment run by the Backyard Scientist on YouTube this year. Given the choice between encountering a live fragmentation grenade on the ground or one in a body of water, you have a better chance of surviving the one on the ground because the odds a fragment hits you may be relatively low. The odds an undissipated shock wave hits you in the water are nearly 100 percent.

One of those things killed the Dallas shooter, but we don't know which. So it's useful to consider what the explosives may have been.

Barnett figured that the Dallas police might have three sorts of explosives at their disposal: flashbangs, countercharges and water devices.

A flashbang is the sort of thing you've probably seen in the movies. "These are designed to disorient people and disrupt in a hostage situation where they don't want to kill anybody, they just want to incapacitate them for a short time," Barnett said. A flashbang is often flash powder in a cardboard tube, designed to make a loud, disorienting flash of light. (Flash. Bang.) This could kill you if it were directly next to your head, for example, but it's not going to generate deadly fragments by itself.

Countercharges are much more powerful. These are "usually a block of C4 or a very fast explosive that they would put on something like an IED or a booby trap where they want to blow it up or destroy it," Barnett said.

A water device is used in a similar way, but focuses the energy of the blast by pushing water forward at a high speed, which is used to incapacitate a device without blowing it to smithereens.

That's the problem with the countercharge scenario: Things get blown to smithereens. "That would be like killing a fly with a hammer," Barnett said. "If I took a block of C4 in there, or high explosives, there's a good chance I'm going to not just damage this guy really badly, but I'm also going to probably disrupt the building in a really bad way." Setting off C4 in a brick-front parking garage, in other words, is not a terrific idea (particularly if you want to preserve your robot).

It's possible — and probably likely — that death wasn't the intent of the police department's efforts. But all of the devices Barnett described could have that effect, depending on the relationship between the device and the target. "If it was six inches from him? Sure," Barnett said. An explosion in a confined space (if the suspect were in a small room, for example) has a larger effect, and an explosive can cause fragmentation of other objects in the vicinity, even if it isn't encased in metal itself.

In other words, it's impossible to know simply from speculation how the Dallas police department killed the shooter in Thursday's attacks. All we can say with certainty is that the encounter was fatal — and that a tactic used by troops in Iraq has made its way to law enforcement in the United States.