Fully autonomous drones killed Russian soldiers during a battlefield test two years ago, according to a Ukrainian drone manufacturer. If true, the incident would represent another milestone in a war that has spurred unprecedented developments in military drones, robots, and AI-guided weaponry.

The one-time test was revealed by Alexander Kokhanovskyy, CEO of the Ukrainian drone maker Aero Center, during an interview with New Scientist at a press event hosted by the Ukrainian embassy in London. Kokhanovskyy described the test using quadcopter drones that were preprogrammed to fly to a front-line area before activating an AI-powered “Terminator mode” that would seek out and attack any target in the given area.

There was apparently no video feed or anything else to show what the “Terminator” drones targeted and attacked. But Kokhanovskyy told New Scientist that human-piloted drones sent to check out the aftermath found “a couple” of dead Russian soldiers, which led to the conclusion that the fully autonomous drones had killed them.

A Ukrainian military commander told New Scientist that his drone pilots only use semi-autonomous systems that always have humans making crucial control decisions. He also described Ukraine’s commitment to “international humanitarian law” while emphasizing that the military always exercises “great care in decision-making in order to prevent civilian casualties.”

The one-time nature of this experiment makes sense when considering the practical limitations of this approach, along with considerations regarding international humanitarian law. Sending fully autonomous drones to attack anything and everything in a given area without any human operator intervention requires careful preplanning and carries the risk of so-called “friendly fire” incidents or attacks on civilian noncombatants. It is also unclear how effective these fully autonomous quadcopter drones were in selecting and attacking targets compared to human drone pilots.