L'Inconnue du Maghreb

Storyline

A name : Taouss Zahira, at the bottom of a letter mailed in Settat, Maroc. 
A date : 24 January 1990. 
Recipient : Charles Bordes, living in Montpellier with his family since the exile of the Pieds Noirs (name given to French nationals born in Algeria) in 1962. He hid the letter in his safe-deposit box at the bank. 
Upon his death, the letter surfaced, provoking quite a stir in the family. Why did Charles not destroy the letter, carrying his secret to the grave?
Who is the person who signed the letter?