The killer of Franz Ferdinand were a 6 assassin group co-ordinated by Danilo Ilić, with Gavrilo Princip given direct credit.
The political objective of the assassination was to break Austria-Hungary's south-Slav provinces off so they could be combined into a Greater Serbia or a Yugoslavia. The assassins' motives were consistent with the movement that later became known as Young Bosnia. At the top of these Serbian military conspirators was Chief of Serbian Military Intelligence Dragutin Dimitrijević, his right hand man Major Vojislav Tankosić, and Masterspy Rade Malobabić. Major Tankosić armed (with bombs and pistols) and trained the assassins, and the assassins were given access to the same underground railroad that Rade Malobabić used for the infiltration of weapons and operatives into Austria-Hungary.
The assassins, the key members of the underground railway, and the key Serbian military conspirators who were still alive were arrested, tried, convicted and punished. Those who were arrested in Bosnia were tried in Sarajevo in October 1914. The other conspirators were arrested and tried before a Serbian kangaroo court in French-occupied Salonika in 1916–1917 on unrelated false charges; Serbia executed the top three military conspirators. Much of what is known about the assassinations comes from these two trials and related records.
Assignment of responsibility for the bombing and murders of 28 June is highly controversial because the attack led to the outbreak of World War I one month later. An evidential approach must be taken to weed through the various claims and counter-claims concerning responsibility.
LOL, I totally read this title as "The Killers OF Franz Ferdinand". Franz being the Archduke of Austria, whose assassination is seen as the key event that precipitated WWI ... and so the answer for all you history nuts:
Gavrilo Princip, a member of Young Bosnia and one of a group of assassins organized by The Black Hand.