A recent Supreme Court ruling is forcing the FBI to deactivate its GPS tracking devices in some investigations, agency director Robert Mueller said Wednesday.

Mueller told a congressional panel that the bureau has turned off a substantial number of GPS units and is using surveillance by agents instead.

"Putting a physical surveillance team out with six, eight, 12 persons is tremendously time intensive," Mueller told a House Appropriations subcommittee. The court ruling "will inhibit our ability to use this in a number of surveillances where it has been tremendously beneficial."

Mueller declined to say how many devices were deactivated. The FBI's general counsel said at a law school conference two weeks ago that the FBI has 3,000 GPS devices.

In January, the Supreme Court unanimously agreed to bar police from installing GPS technology to track suspects without first getting a judge's approval. On Wednesday night, the FBI said many of the GPS trackers were placed with court authorization and so were not deactivated.

"We have a number of people in the United States who we could not indict, there's not probable cause to indict them or to arrest them who present a threat of terrorism, articulated maybe up on the Internet, may have purchased a gun, but taken no particular steps to take a terrorist act," Mueller said. "And we are stuck in the position of surveilling that person for a substantial period of time."