John D. Lee in private journal testifies that Brigham Young did not know about the massacre until after it happened and would have tried to stop it if he could.
John D. Lee, Journal, ca. July 20, 1871, rep. in Robert Glass Cleland and Juanita Brooks, eds., A Mormon Chronicle: The Diaries of John D. Lee, 1848–1876 (San Marino, CA: The Huntington Library, 1955), 2:164
(About July 20th, 1871), I had a lengthey interview with the reverend Mr. Braun, one of the advocates of young Joseph. . . . But let this, My Testim[on]y, suffice from troubling Me on the subJect any More: Brigham Young knew nothing of the Mountain Massacre until it was all over; & verry Much regretted it when he heard of it, & I am satisfied that he never would have suffered it, could [he] have preventd.