The Black Hand secret society was responsible for killing Archduke Ferdinand that started World War One one month later, please scroll down link to "IV. THE SEAL AND THE OATH OF ALLEGIANCE".
Article 34. The Organisation's official seal is thus composed: In the centre of the seal there is a powerful arm holding in it's hand an unfurled flag on which there is a Skull and Bones!!!
Constitution of the Black Hand - World War I Document Archive
Masonic tracing boards show the skull and cross bones, link below:-