FBI Director Christopher Wray told a Senate panel Wednesday that he expects that terrorists would be using unmanned aerial vehicles in attacks against the U.S. in the near future.

Terrorists are using drones for attacks in other countries, he said, so it is only a matter of time before they are used in the U.S.

"It is a topic that we are discussing a lot lately. I think we do know that terrorist organizations have an interest in using drones. We have seen that overseas already with some frequency. I think that the expectation is that it is coming here, imminently," Wray said at a hearing of the Senate Homeland Security and Governmental Affairs Committee.

Federal agencies are working on counter-measures, but the work is challenging, he added. Drones are "relatively easy" to acquire and operate and their use can be hard to disrupt and monitor.

Elaine Duke, acting secretary of the Department of Homeland Security, said the department had noticed an increase in the use of drones along border states. "They could be used for surveillance, or bringing in illicit materials or they could be used for violence," she said.

Nicholas Rasmussen, director of the National Counter Terrorism Center, said at the hearing that drones have been used by terrorist organizations such as the Islamic State in Syria and Iraq. "There is a community of experts that has emerged inside the federal government that is focused on this pretty much full time. Two years ago this was not a concern. ... We are trying to up our game," Rasmussen said.

There still seems to be some kinks in the effort, though. Asked by Sen. John Hoeven, R-N.D., who in the government was coordinating the activities, Rasmussen said that while he was participating in the anti-drone efforts, he didn't know who was in charge. "I will get you an answer on that," Rasmussen told Hoeven.