Officers investigating the incest and sexual abuse case against Josef Fritzl revealed yesterday that he started building the underground dungeon in which he imprisoned his daughter six years before her incarceration began.

Speaking to The Times at a press conference yesterday, Colonel Franz Polzer said that Mr Fritzl had planned the imprisoning and sexual abuse of Elisabeth in astonishing detail.

“Fritzl acted with premeditation when he began building the underground cellar of his home in 1978,” he said. “We assume he had already selected his daughter Elisabeth, who was to become a prisoner of the concrete dungeon.”

Mr Fritzl, 73, is accused of imprisoning his daughter, now 42, in a purpose-built dungeon beneath his house, where he sexually abused her as she gave birth to seven of his children.

Commentary: a society that nurtures secrets

What society would accept the probability of three children being left on the doorstep of Josef F and grant him adoption?

Multimedia

• Pictures: house of horror

BACKGROUND

• Why are child abuse scandals happening in Austria?

• Analysis: damage caused may never be healed

• Brutal double-life of Josef Fritzl emerges

• Police search other Fritzl properties in Austria

Related Links

• Closed doors? Should be more of it

• Fritzl 'insanity plea' to dodge prison

Mr Polzer also revealed that investigators, who have so far been focusing on examining the scene of the crime, Mr Fritzl’s three-storey family house in the town of Amstetten, have found a second entrance to the dungeon.

The 500kg iron and concrete door leading to the second entrance was so well concealed and difficult to open that police had to use heavy machinery borrowed from the local fire brigade. The door was hidden in a room beneath the stairs that could be approached from the garden. It weighs about 500kg and is only one metre high and “so difficult to detect, that it is reminiscent of something from the Harry Potter books”, Mr Polzer said.

Police believe, however, that the second entrance had not been used recently because it was so heavy that it had become impossible to open.

They confirmed earlier reports that Elisabeth was locked up for nine years, from 1984 until the birth of her fourth child in 1993, in a single room of only 35 square metres, with an improvised shower and toilet. Mr Fritzl then expanded the dungeon with an additional two rooms to create a space totalling 55 square metres. It suggests that her children would witness Mr Fritzl’s sexual assaults on their mother.

Police said that anyone trying to access the dungeon would need to pass through eight doors; all were locked and the last two were electronically controlled. The second entrance was secured by four doors. “Fritzl planned every detail with astonishing precision. His diabolical plan was almost perfect,” Mr Polzer said.

Although police have established that Mr Fritzl’s son Josef, 37, who lived with his parents, had the key to two of the doors to access the boiler room, they denied that he could have known about the dungeon because only his father held all eight keys.

“There is no reason to believe that this very energetic man, who had focused all of his energy on his unfathomable plan, would involve anyone in the realisation of his perfect crime,” Mr Polzer said.

Bernhard Kepplinger, head of the hospital where the Fritzl family is being treated, said that Elisabeth, and her children Stefan, 18, Lisa, 15, Monika, 14, and Alexander, 12, as well as Mr Fritzl’s wife Rosemarie, 68, were in good condition.

Dr Kepplinger said: “The family are developing a good social interaction. The mother and the grandmother are preparing the meals together. The skin of the mother and the three children that have not been exposed to sunlight is now assuming a normal colour.

They are still oversensitive to sunlight but are improving.”

Dr Kepplinger also said that during their years in the cellar, the family was supplied with vitamin D supplements and even a UV lamp by their father, who also installed an aquarium with goldfish in their dungeon.

The condition of the oldest daughter, Kerstin, who is in an artificially induced coma, is said to be no longer life-threatening.

Legal experts said that the children Mr Fritzl fathered with his daughter could sue him for millions in damages, when it emerged that their therapy will cost more than €1 million (£780,000).

Times Recommends

• Please help: maimed children of war want their lives back
• Cabal seizes diamond riches to buy power
• White House gatecrashers got to the president