Background
Devices exist which, by sending an electrical impulse of ultrasonic frequency through a telephone circuit by tapping, will permit remote surveillance of any office in which a telephone is located by utilizing the telephone microphone as a pick-up. Such eavesdropping may be accomplished without additional wires and is possible even though the handset is in a “hung-up” position.
Below, you will find thousands of pages archived from the FBI on ultrasonic listening devices, wiretapping, violations, and countermeasures.
Document Archive
Ultrasonic Listening Devices / Wiretapping, 1945 – 1989 (80-HQ-760) – Release #1 – [1,064 Pages, 200.1MB]
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