When a police robot killed suspect Micah Johnson in Dallas early Friday morning, it was likely an unprecedented event. But according to Steve Ijames, recently retired assistant chief of police in Springfield, Missouri, and a recognized expert in SWAT tactics, it was not a watershed moment portending a weaponized robotic future. The standoff after the police massacre at a Black Lives Matter protest was unique for a number of reasons, he says. And it was likely the only choice the police had.

The police department hasn't elaborated on the device the bomb-defusing robot used or how exactly it killed the suspect, nor on the circumstances that led to the decision. But Ijames explains that a number of factors would have determined their novel use of the robot.

Line of Sight

Although details about the incident are still scarce, Dallas Police Chief David Brown said this morning that his department had brought in the robot only after "negotiations broke down" with the suspect. "We had an exchange of fire with the subject. We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was," Brown said.

Ijames interprets this to mean the Dallas police could not see Johnson. "I think you'll learn as this plays out that this person was secreted in a location that they couldn't see him," Ijames says. "If they could see him, they just would have shot him with a rifle, no question. And looking at the physical location—the concrete and the chance of ricochet—they probably didn't want a lot of bullets flying around. They were probably trying to limit their fire as best they could."

Since the Dallas police chief mentioned an explosive device killed the suspect, Ijames says that his years of SWAT experience leads him to believe that the suspect was barricaded behind or in something the police needed to breach. This would have required the team to create an explosive strong enough to open whatever stronghold the suspect was in.

"You'd go to the master breacher and say I want to wheel the robot in there, give him a chance to surrender, and if he doesn't I want to set off a charge that will absolutely put him on the deck, dead or alive," Ijames says. They may have been communicating with him via video feed, warning him to put down his gun before they decided to detonate the blast.

How a Police Robot Might Kill

Ijames says there are two different ways the robot may have killed Johnson, directly or with a secondary blast.

Robots used to defuse bombs have a gun—some are 50-caliber, some are 12-gauge guns—that shoot a payload at the bomb; usually water, he says. "A column of water shot like a rifle bullet ... at high velocity, blows things up. They could very well have shot him with one of those and it would have sure killed him," Ijames says.

But police often breach doors in a similar way, which would be necessary if Johnson was barricaded behind something. That's the second possibility. "A lot of breaching done in law enforcement is done with what's called a water-impulse charge," he says, "where you literally take something like an IV-size bag of normal saline water and set an explosive charge on one side and you would smack the door with the high-speed water." If this is what happened, Johnson was either killed by the overpressure from that explosion or injuries caused by projectiles launched at high speed.

To illustrate the speed, Ijames says a 9mm round from a police handgun soars at about 1,100 feet per second. The rifle the suspect was shooting last night, he says, sends a bullet flying at about 3,000 feet per second. An explosive charge used to cut open a door or wall would cause the door material to travel at more than 25,000 feet per second. Those flying projectiles would have been enough to kill Johnson.

But so could the pressure from the blast alone. When a supersonic blast of air from an explosion hits a person, it forces liquid inside the body to slam from the front to the back of the body at the same speed as the blast of air. Ijames says this causes internal organs to break.

A Rare Event?

So, is this the first of many robot cops killing suspects? Critics worry that what happened in Dallas could open the floodgates to more robotic police killings.

“[B]ecause ground robots may allow deadly force to be applied more safely and easily, they raise the danger that they will be overused," Jay Stanley, policy analyst with the American Civil Liberties Union told WIRED. "Remote uses of force raise policy issues that should be carefully considered and addressed by our society as technology advances and should remain confined to extraordinary situations.”

Military expert Peter Singer says that though there's no doubt law enforcement will increasingly rely on robots, the question of whether they will or should be weaponized remains open. "This is a parallel to the Google [self-driving] car issue," Singer says. "There are things that the technology can do. Now we’re into the debate not about could, but should it?"

Ijames, however, believes that regardless of the debate, lethal police robots will be applied in the future very sparingly. Last night, he says, was not a threshold event but a rare situation in which a unique set of stars aligned. "The reason you have not seen many things like this is that scenarios historically just do not get to that extreme level. There are just other ways to resolve it. We almost always get sights on [the suspect] and shoot him," he says.