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.
Ultrasonic Listening Devices / Wiretapping, 1945 – 1989 (80-HQ-760) – Release #1 – [1,064 Pages, 200.1MB]
Ultrasonic Listening Devices / Wiretapping, 1945 – 1989 (80-HQ-760) – Release #2 – [8 Pages, 2.0MB]
Countermeasures (80-HQ-805) – Release #1 – [2,666 Pages, 121.8MB]