Be flexible with your keywords. Expand on the word riot (synonyms) and try using keywords describing the instigation of the riot. For example: personal name or an event associated with the riot
Check the public library websites located in the city/county/state where the riot occurred. Public libraries and state archives often collect digital photographs, newspaper articles, interviews, and audio/video recordings covering historic events within their geographic area. Call or email the public library closest to the riot area for further information.