London, May 12 : Dungeon dad Josef Fritzl's 18-year old son Stefan, whom he held captive with his daughter Elisabeth, might provide evidence that could convict the sex-beast of murdering one of the twins he produced by raping his daughter.
12 years ago, Stefan saw his mother delivering twins, one of whom, Michael, died after only three days in Fritzl's damp dungeon and was later burnt in a furnace by the cellar monster.
Prosecution spokesman Dr Gerhard Sedlacek said that they wanted to question Elisabeth as well as other children who were held captive by Josef.
"We want to question the mother and the children in the cellar at that time. The key question is whether the child would have survived with proper medical attention," Mirror quoted prosecution spokesman Dr Gerhard Sedlacek, as saying.
He added: "If Fritzl knew his daughter was about to give birth and did not help her then he'll be charged with murder on the grounds of negligence. That carries a life sentence - and in Austria, life means life."
Fritzl has already confessed to his lawyer Rudolph Mayer that he was aware that Elisabeth was to deliver, but he left her on her own with only medical manuals at her disposal.
He has also admitted that he disposed off Michael's body, but maintains that the child was already dead.
Also to be questioned will be Elisabeth's 19-year-old daughter Kerstin, if she ever recovers from her terrible ordeal as she lies in a critical condition in hospital.
Josef, a former electrical engineer, is accused of seven charges of rape leading to pregnancy, kidnap, assault and neglect of his children. And all these charges carry a maximum sentence of 15 years.