At first, it was believed that the son of MINAMOTO no Yoshitsuna (Yoshitada's uncle), MINAMOTO no Yoshiaki, and his retainer, FUJIWARA no Suekata, were responsible for the assassination.
義忠の暗殺は当初、叔父源義綱の子・源義明とその家人藤原季方の犯行とされた。