Source: Tenmangu goden kiryaku, by Hirata Atsutane (1820) with illustrations by Kuwagata Keisai
The lightning strike on the palace in 930, which killed four courtiers including Fujiwara no Kiyotsura, who had participated in faking the charges against Michizane. This incident led to Michizane being identified with the god of lightening and thunder, Kaminari.
[information from Robert Borgen, Sugawara no Michizane and the Early Heian Court, p. 308]