That mystery was partly solved last month by Jürgen Matthäus, who recently retired from his post as the head of research at the US Holocaust Memorial Museum in Washington, D.C. According to an article he wrote for the September issue of The Journal of Historical Studies, a German-language publication, the killer was Jakobus Onnen, 34, a former teacher from the town of Tichelwarf, near the German border with the Netherlands. After one of Onnen’s living relatives provided Matthäus with family photos, an artificial intelligence tool made the match with 99.9 percent confidence. The photo emerged in 1961, during the Jerusalem...