It is shown that the delay time of a chalcogenide glass threshold switch is not significantly affected by strong illumination, sufficient to increase the conductance by an order of magnitude. This means that the initiation of threshold switching is not due to free carriers within the bulk material. Noise measurements in the pre-threshold region demonstrate the existence of noise, in excess of thermal noise, increasing sharply towards the threshold point. The results suggest impact ionization by carriers originating at the electrodes as the best available hypothesis for the nature of the threshold condition.