The FBI has been facing off against a new menace: drone swarms under the control of criminal groups.

This past winter in a US city, criminal suspects used drones against an FBI hostage rescue team to flush out the federal agents, according to Joe Mazel, the head of the FBI's operational technology law unit.

During the incident, the drones made a series of "high-speed low passes at the agents" to disrupt the rescue team's operation, Mazel said at a conference on Wednesday, according to Defense One.

"We were then blind," Mazel said, describing how the rescue team lost situational awareness. "It definitely presented some challenges."

The criminals not only used the drone swarm to rattle the rescue team, but to also conduct counter-surveillance. "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," Mazel added.

According to Defense One, Mazel declined to offer details of when or where the incident took place. But he said criminal groups are increasingly using drones for their schemes. This can include attempts to intimidate witnesses by sending a drone to surveil police stations to see "who is going in and out of the facility, and who might be cooperating with police," Mazel said.

He also pointed to their malicious use in Australia. Criminal groups will deploy the flying bots to help them monitor shipping containers carrying their illegal goods. If a port authority worker gets too close to the container, the crooks will know and call in a false alarm, like a fire or a theft, to distract the port authority worker away from the container.

The FBI did not immediately respond to a request for comment about the drone swarm incident. But it's also not a complete surprise. Reportedly, drug cartels have also been caught planting bombs on drones or using them to smuggle drugs. In addition, militant groups in Syria, including ISIS, have been weaponizing off-the-shelf drones to drop explosives on US and Russian forces.

This article originally appeared on PCMag.com.