Josef Fritzl, Austria's most infamous prisoner, has been held in psychiatric detention in a high-security unit since he was sentenced to life behind bars in 2009 for his heinous crimes.

He fathered seven children by his daughter Elisabeth while keeping her as a sex slave in the basement of his home in Amstetten. Elisabeth, now 58, was locked in the dark, damp and dingy cellar for 24 years and raped by Fritzl thousands of times, with the horrific story sending shockwaves around the globe.