Solomon Zeitlin, Taḳḳanot ʻEzra (1917)
“ When is a man purged of his uncleanness? After sunset. Ṭebilah alone does not render him pure. As the priest who burns the Red Heifer must be pure, [15] and we are apprehensive lest by accident he come under the head of מקרה לא טהור, or lest his brother priests have touched him, in which case the ṭebilah (ablution) would not have the immediate effect of purging him and qualifying him to burn the Heifer—therefore the Sadducees considered it necessary to defer that burning until after sunset. ”
