It is assumed that each instance would have some kind of documentation. This assumption is justified because these events occured in modern history, in an area with a free press and a highly literate population. We also know from other instances of genocide that evidence abounds, even with the official attempts to suppress it. There are paper trails of official directives and eye-witness accounts of every genocide in modern history.
December 29, 1890 - WoundedKneeMassacre? - Wounded Knee, South Dakota - 300 (est.) people killed.