They recently tried to thwart an FBI hostage rescue, Joe Mazel, chief of the FBI’s operational technology law unit, said this week, according to a report by news site Defense One.

Mazel, speaking at the AUVSI Xponential drone conference in Denver, said that criminals launched a swarm of drones at an FBI rescue team during an unspecified hostage situation near a large U.S. city, confusing law enforcement. The criminals flew the drones at high speed over the heads of FBI agents to drive them away while also shooting video that they then uploaded to YouTube as a way to alert other nearby criminal members about law enforcement’s location.

Mazel didn’t elaborate on when the incident occurred, saying that the case was “law enforcement-sensitive,” the news report said. What was clear, however, was that the drones “definitely presented some challenges,” he said.

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Mazel also said that unspecified criminal groups are using drones to intimidate witnesses. In one example, he said that criminals flew drones near police departments to monitor whether any informants enter or leave the building.

Mazel’s comments highlight the potential for criminals to misuse drones, which continue to rise in popularity. In June, USA Today reported that people are using drones to sneak illegal substances like drugs into federal prisons. That USA Today report followed a similar report in 2016 by the BBC that also focused on prisoners trying to use drones to smuggle contraband in U.K. prisons.

Fortune contacted the FBI for more information and will update this story if it responds.