An investigation into a racist photo on Virginia Gov. Ralph Northam’s 1984 medical school yearbook page didn’t determine who was in the photo, a report on the investigation said Wednesday.

Eastern Virginia Medical School commissioned the independent probe, led by law firm McGuireWoods LLP, in early February after the picture resurfaced, sparking turmoil in Richmond, the capital. The photo depicts one person in blackface and another in Ku Klux Klan attire.

Mr....