Police used an unusual weapon to kill a man suspected of murdering five officers in Dallas on Thursday: A robot.
Dallas Police Chief David Brown told reporters during a press conference that police officers cornered a suspect and attempted to negotiate with him until the talks devolved into gunfire. Because of the danger to officers, Brown said law enforcement “saw no other option” but to use a robot carrying a bomb to kill the suspect.
Several legal and warfare experts said on Friday on Twitter that the incident marked the first time U.S. law enforcement used a robot to kill someone.
Many details are unavailable about how exactly the Dallas police used the robot beyond what Brown told reporters. However, researchers at The Center for the Study of the Drone at Bard College have used publicly available information to conclude that Dallas police have at least three so-called unmanned ground vehicles, or robots, obtained in April 2014 through a federal program that lets law enforcement buy surplus military equipment.
Law enforcement can buy anything through the Defense Department’s 1033 surplus equipment program, from armored vehicles and firearms to robots and gym equipment, explained Arthur Holland Michel, the center’s co-director.
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Dallas police spent $10,000 for each robot, according to the Defense Department data. Additionally, a 2008 public city document shows that Dallas police sought to buy a robot that would not exceed $207,671. Remotec, that built the robot, is a subsidiary of Northrop Grunman (NOC) and makes a line of robots called ANDROS that are used to handle and dispose of hazardous materials.
Michel explained that Dallas police may have used one of two types of robots against the suspect—the ANDROS Robot or another robot called the MARCbot. There’s always a possibility that another type of robot could have been used, it should be noted. Both models have been used by the military for bomb disposal, but were not designed for use against humans, he said.
The MARCbots, which stands for multi-function, agile, remote-controlled robot, have wheels, and can weigh around 30 pounds. They can cost several thousand dollars, which is far cheaper than the ANDROS Robots that can cost over $100,000 and weigh up to 200 pounds, Michel explained.
These bomb disposal robots are also typically outfitted with cameras, which allow operators to see from the robot’s perspective. In the case of the Dallas shootings, this camera feature would be useful to police so they could guide the robot near the suspect and set off an explosion rather than throwing a grenade in his direction.
It’s important to note that both of these robots are not autonomous and therefore require a human to remotely direct their movements. Although technology has advanced to the point where robots can move freely in very specific circumstances and environments like a warehouse, they cannot operate independently in unfamiliar locations where there are too many variables like poor lighting and too many people moving around.
Like other experts, Michel believes this is the first time a U.S. police department used a robot to harm someone. He said that the use of robots is a growing trend in law enforcement, but this Dallas incident marks the first time he’s heard of robots being outfitted with weapons to kill criminals.
The military has previously strapped land mines to MARCbots during the second Iraq War to battle insurgents. But like the Dallas incident, this was not what the robots were designed to do, he explained.
A key question that needs to be answered is whether Dallas police came up with the idea to use a robot to kill out of desperation, or whether they had previously devised such a plan and decided to finally carry it out.
For more about the Dallas shooting, watch:
“This took me by surprise,” Michel said. “I have not encountered any material that indicated that police departments were planning to use unmanned ground vehicles in this way.”
Because there is “no precedent in the [history of] domestic law enforcement” of using robots to kill, their use by Dallas police is likely a turning point in the use of robotics and other advanced technologies by police. The next time the public learns of a police department acquiring a robot “it will be in the context of what happened last night,” Michel said.
The use of the robot “got the job done, in a sense” and perhaps saved the lives of police officers, he added. However, the public may question “if they want potentially weaponizable robots in their local sheriff’s arsenal,” Michel concluded.
Fortune contacted the Dallas Police Department for more information and will update this story if it responds.