De Koven was the first of six victims of German-born serial killer Eugen Weidmann, also known as "Karrer", a gang leader who confessed to his crimes and had as many as nine accomplices.
Police could not locate the contact man even though he advertised frequently in the Paris edition of an American newspaper.
By September, Sackheim offered a reward requesting information which would lead to the finding of De Koven's presumed abductors.
Weidmann's other victims were realtor named Raymond Lesobre, theatrical producer Roger LeBlond, a chauffeur Joseph Couffy, a young German he met in jail Fritz Frommer, and private nurse Janine Keller.
Police traced him to an expensive dance bar, the Pavillon Bleu, in La Celle-Saint-Cloud, about a 15-minute walk from his villa.
Bloom knew De Koven from her youth and remarked "there was something fine, distinctive, and superior about Jean, even in her childhood.