The National Police Agency ordered other police forces in 2006 to keep their use of GPS tracking devices in investigations secret and to not mention them in case reports, it has been learned .
The revelation surfaced Wednesday amid a flurry of civil cases where questions were raised over the legality of police using GPS devices to track people's movements without a court warrant. The nation's courts have been divided by the issue.
The Japan Federation of Bar Associations said in a statement that GPS should be used with a warrant, adding that the method tends to "significantly" violate people's privacy by allowing police employees to easily and cheaply track people for long periods of time without their knowledge.