Firstly, there is an obituary that appears in "Shoku Nihongi" (Chronicles of Japan), showing that he died because the ghost of Fujiwara no Hirotsugu holding a grudge against him killed him.
まず史書である『続日本紀』の彼の卒伝に、「藤原広嗣が霊の為に害せらる」とあり、当時広嗣の怨霊によって彼が死んだとの考えがあったことが知られる。