Franz Polzer, of Lower Austria's criminal police said 73-year-old Fritzl, who has confessed to holding his daughter, Elisabeth, captive for 24 years and fathering her seven children, appeared to have had an "obsession" about locking her up that began at least six years before her underground ordeal began. "He planned her incarceration and prison in minute detail with the objective of keeping his daughter captive for a long time," said Mr Polzer. "This goes some way towards explaining how he was able to conceal his actions from the rest of the world for so long."

Mr Polzer said that, in 1978, Fritzl had applied for planning permission to build an extension to his cellar in Amstetten. At the time Elisabeth was 12. He said detailed police investigation of the cellar had revealed a labyrinth of chambers. "It is hard to imagine, but anyone wanting to get into the prison from outside has to pass through a total of eight doors," added Mr Polzer.

Fritzl started raping his daughter when she was 11. But in 1984, when she was 18 and had once run away from home, he decided to imprison her indefinitely. Statements given to police by Elisabeth Fritzl say he lured her beneath ground by pretending he needed help to carry a door down into the cellar for storage.

Once there he drugged his daughter with ether and dragged her into a small chamber where she was handcuffed to a pole for two days. Fritzl later put her on a leash made from an electric cable, which enabled her to reach a lavatory.

For the first nine years of her ordeal, Elisabeth Fritzl was held prisoner in the same cell. She was raped by her father every three days on average and gave birth to three of their seven children in the same room. Fritzl extended the complex in 1993 by constructing a corridor to other rooms, one of which contained a double bed and a television.

Police had only recently discovered the existence of the second massive steel entrance hatch that weighed approximately half a tonne, said Mr Polzer. "The quality of the air underground leaves much to be desired," he said.

Austria's worst recorded rape and incest case came to light on 19 April after Elisabeth's Fritzl's 19-year-old daughter Kerstin, who spent her whole life underground, became seriously ill and was finally allowed by Josef Fritzl to be admitted to hospital. Kerstin Fritzl has been in an artificially induced coma ever since.

Doctors caring for Elisabeth Fritzl and the two other children freed from the cellar said yesterday that although they were making a gradual recovery, Elisabeth and her five-year-old son Felix had suffered a relapse due to over exposure to daylight. "We had been opening their curtains more and more to get them used to daylight," said Dr Berthold Kepplinger, head of the clinic where they staying. "But they have had a bit of a setback. We have now given them back their protective glasses," he added.

Dr Kepplinger said Fritzl had fed his prisoners with tablets containing vitamin D and had even arranged for a sun lamp to be set up in their windowless cellar to stop them contracting rickets. "They also had an aquarium in the cellar," he added. "We have installed a new one for them in their accommodation in the clinic and the youngest one has been given back his teddy. They are all benefiting from the fresh air and balanced diet."