Board Thread:Theories and Speculation/@comment-28323245-20160509192301/@comment-732150-20160509202931

Fourth databook already said that Indra is the one who first changed ninshu into ninjutsu. All we know about him and BZ is that BZ was the one who manipulated him into fighting Asura. There's no word about the details and specifics of BZ's manipulation, but the databook directly credits Indra with the development of ninjutsu.