Yesterday, judges ordered that Fritzl, 73, who was remanded in custody at the end of April, should be detained for a further month while Austrian prosecutors complete the list of charges they intend to press against him.

Fritzl has confessed to holding his daughter, Elisabeth, hostage underground from the age of 18 onwards and to fathering her children by raping her almost once every three days because he was sexually obsessed with her. But he has also told police that one of his children, a baby twin named Michael, died of a mystery illness.

He told police – in his first confession to them after his arrest – that the boy was born, like all the other children, without receiving any medical attention, on 28 April, 1996. Fritzl said he died three days later and has admitted that he incinerated the baby's corpse in the wood-fired boiler that supplied his heating system to "get rid of it".

Gerhard Sedaleck, a spokesman for the Lower Austrian state prosecutors' office, said yesterday that Fritzl would be formally questioned over the next few days. "If we establish that he could see that the child was severely ill but still failed to summon help, then there are grounds for bringing murder charges against him on the basis of criminal negligence," he said.

Alexander, the dead baby boy's twin brother, is now 12. Shortly after his birth he was deposited upstairs in Fritzl's "normal" home above the cellar bunker where he was looked after by Fritzl and his wife, Rosemarie, together with his sisters Lisa, 15, and Monika, 14.

The children were unaware that they had a sister and two other brothers – Kerstin, 19, Stefan, 18, and Felix, five – who had spent their entire lives underground. The three cellar children saw daylight for the first time when they were released.

An inkling of the horrific circumstances Elisabeth, now 42, and her children were forced to endure has been provided by Fritzl in an interview he gave to the Austrian media via his lawyer from his prison cell. His account of his life in the cellar was a bizarre attempt to counter the media image of him as a monster.

He admitted that he never summoned a doctor to the underground prison and that all of the children who resulted from his incestuous relationship with his daughter were born without any proper medical assistance.

It appears that Elisabeth Fritzl was forced to give birth to every one of her children on her own. Fritzl said he gave her "medical books, nappies, disinfectants and towels" in an attempt to familiarise her with childbirth.

Elisabeth was never allowed to see a dentist and Fritzl admitted that, during her 24-year incarceration, all of her teeth went rotten and "fell out one by one".

Fritzl was largely oblivious of the suffering of his prisoners. Although Kerstin, 19 started developing screaming fits and a form of epilepsy and his youngest son Stefan, aged five, shook uncontrollably, he responded by giving them aspirins.

At the end of April, Kerstin lapsed into a coma and Elisabeth finally persuaded Fritzl to take her to hospital, where doctors notified the police for the first time.