A fiber optic network has alarmed fiber optic lines in the cables connecting a secured junction box to plural user lock boxes. An outgoing alarm line and return alarm line in each cable connect the junction box to each user box. The outgoing alarm line is looped to the return alarm line inside the user lock box. The return alarm line is looped to the outgoing alarm line of a different cable inside the junction box to interconnect a plurality of alarm lines passing through a plurality of user boxes. A detector detects an alarm signal in the connected alarm lines to trigger an intrusion alarm. Power to the components in the box is disconnected when the box is opened and ventilation openings to the box are closed when the box is closed.