The case against Josef Fritzl, who imprisoned his daughter in a basement for 24 years, suffered a further blow yesterday as it emerged that some of his children have apparently refused to testify against him. He could now be facing as little as ten years in jail.

Mr Fritzl, 73, is accused of raping and incarcerating his daughter Elisabeth, 42, in a purpose-built concrete bunker in the cellar of his home in the town of Amstetten, where he fathered seven children with her. He is also charged with causing the death of his infant son, a baby called Michael, who died soon after birth. Mr Fritzl is alleged to have burnt his body in an incinerator.

Elisabeth Fritzl has finished giving evidence to the court authorities and while details of her testimony have not been made public, it is believed that she accused her father of rape and psychological torment, as well as for the death of her child, who is thought to have died three days after birth because of conditions in the cellar.

Prosecutors told The Times that Ms Fritzl’s two other adult children, who were supposed to provide key testimonies against their father and support their mother’s allegations, were likely to refuse to give evidence.

The prosecution spokesman, Gerhard Sedlacek, said: “We still have not fixed a date for the questioning of the two adult children but it has now emerged that they could make use of their right not to speak to the authorities and refuse to give evidence against their father.

“The matter will be discussed between their lawyer and the judge in charge, but it has been suggested that they \ decline to give any statement.”

Mr Fritzl, a retired engineer, has made a partial confession and is facing charges of manslaughter for the baby that died, as well as rape, coercion, deprivation of freedom and incest. However, prosecutors said that the first two charges would be very difficult to prove because of the lack of scientific evidence and because they were relying to a large extent on the testimonies of the children to strengthen their case.

If his adult children refuse to give evidence, Mr Fritzl could be facing a ten-year prison term as Austrian law, unlike British law, does not allow for multiple convictions. This means that even if he is found guilty of several crimes, he will only serve one punishment, for the offence that carries the longest prison sentence, which in this case is likely to be deprivation of freedom.

The adult children of Mr Fritzl and Elisabeth — a girl aged 19, and a boy, 18 — were not allowed to leave their cellar and were imprisoned for the whole of their lives, with their mother and brother aged 5. They were freed by police on April 26.

The other three siblings, aged 15, 14, and 12, were allowed to live with Mr Fritzl and his wife, Rosemarie, 68. As they are under-age they are not allowed to testify under Austrian law. Mrs Fritzl denies any knowledge of her husband’s actions. After two months of medical and psychiatric treatment in a clinic, the family have been moved to an apartment at a secret location.

Mr Sedlacek said that it was “extremely difficult” to prove the manslaughter charges because so much time had elapsed since the death in 1996 of Michael, who was the twin brother of Alexander.

“There is no direct forensic evidence due to the time elapsed and there is no body, since he allegedly burnt the baby in an oven. The charges could therefore only be based on the testimony of his daughter. In addition, in order to stand up the charges of manslaughter, one would need to attain evidence that there was premeditation, as well as evidence that the child would have survived had it received medical attention.”

The prosecution has hired a paediatrician specialising in newborn babies to try to provide evidence for the charges. According to Austrian law, the maximum sentence for manslaughter is life imprisonment.

The allegations of rape, which carry a maximum sentence of 15 years in prison, would also be difficult to prove because of lack of corroborating evidence, and the charges would only be based on the testimony of Elisabeth.

Ms Fritzl’s testimony was videorecorded and will be presented to the court during the trial, expected to take place by November, to honour her demand that she and her children never be confronted with their father again. Mr Fritzl has waived his right to follow the questioning of his family and ask them questions via video-link.

A jury of eight will decide whether Mr Fritzl is guilty. If he is convicted they will confer with a panel of three judges to determine his sentence.

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