Board Thread:Consensus Track/@comment-30918569-20200906013106/@comment-25319479-20200906023425

In the anime it was most likely Yagura, as he was Mizukage at the time of Juzo's death which was prior to Zabuza becoming a Seven Ninja Swordsmen, and at the time of the time skip chunin exams Yagura is suggested to have passed away shortly ago. Saying that, the assassination attempt could have always been on a retired Mizukage such as the Third, making it still unknown in the anime.