Metin Kaplan, the self-named "Cologne Caliph," who was jailed following his extradition from Germany back in 2004, was released yesterday by an Istanbul court which has ordered his retrial on terror charges.

Kaplan was extradited by Germany to Turkey after succeeding his father Cemalettin Kaplan as leader of the so-called Anatolian Federal Islamic State.

He was initially sentenced to life in prison on terror charges, including a plot to bomb Anıtkabir, the mausoleum of modern Turkey's founder Mustafa Kemal Atatürk and occupation of a mosque in Istanbul. In 2010, his sentence was reduced to 17 years in prison.

His lawyers filed an appeal to the court in 2014 for a retrial and release, arguing that Kaplan was not fit to stay in prison due to his deteriorating health.

Kaplan was the leader of the Kalifatsstaat or Caliphate State, an organization active in Germany that made the headlines for its rally with toy guns in a sports hall. He was first arrested in Germany on charges of masterminding the murder of Halil İbrahim Sofu, another member of the group allegedly poised to take over the leadership of the organization.

During his trial in Turkey, he repeatedly denied that he had planned any acts of violence, although he admitted his desire for the establishment of a Sharia state in Turkey.