Last winter, on the outskirts of a large U.S. city, an FBI hostage rescue team set up an elevated observation post to assess an unfolding situation. Soon they heard the buzz of small drones — and then the tiny aircraft were all around them, swooping past in a series of “high-speed low passes at the agents in the observation post to flush them,” the head of the agency’s operational technology law unit told attendees of the AUVSI Xponential conference here. Result: “We were then blind,” said Joe Mazel, meaning the group lost situational awareness of the target. “It definitely presented some challenges.”

The incident remains “law enforcement-sensitive,” Mazel said Wednesday, declining to say just where or when it took place. But it shows how criminal groups are using small drones for increasingly elaborate crimes, Defense One reports.

Mazel said the suspects had backpacked the drones to the area in anticipation of the FBI’s arrival. Not only did they buzz the hostage rescue team, they also kept a continuous eye on the agents, feeding video to the group’s other members via YouTube. “They had people fly their own drones up and put the footage to YouTube so that the guys who had cellular access could go to the YouTube site and pull down the video,” he said.

Mazel said counter surveillance of law enforcement agents is the fastest-growing way that organized criminals are using drones.