We analyze synchronized high speed video images and ELF-VLF radio emissions from 11 sprite clusters observed on 6 October 1997. Quantitative analysis shows that vertical lightning charge moment changes of 150--1100 C⋅km occurred before the optical emissions reached their peak with delays of 2--11 ms from the lightning discharge. This threshold unexpectedly decreases with increasing delay from parent lightning to peak emissions. We also find that sprite charge moment change and minimum sprite altitude are not well correlated with the vertical charge moment change in the parent discharge. These observations do not agree well with present sprite generation models, and we suggest that streamer development and horizontal lightning charge motion can play a significant role in sprite generation. ¿ 1999 American Geophysical Union