Reporter: Peter Harvey
Producer: Howard Sacre
You have to shake your head in disbelief - how on earth can these things happen?
And in this particular case, how could he possibly get away with it for so long.
It's just unthinkable - a father, who kept his daughter locked away in a secret chamber for 24 years.
The poor girl was just 18 when the horror began, and over the years she had seven children.
One died, three were kept prisoner down in the cellar.
And, unbelievably, the others were adopted by the father and lived openly upstairs as his "grandchildren".
No wonder the family's story stunned the world. And we must add a warning - what follows contains strong adult themes.
Blog: Peter Harvey unmasks the monsterous world of Josef Fritzl
INTRODUCTION PETER HARVEY: You have to shake your head in disbelief. How on earth can these things happen? And in this particular case, how could he possibly get away with it for so long. It's just unthinkable - a father who kept his daughter locked away in a secret chamber for 24 years. The poor girl was just 18 when the horror began, and over the years she had seven children. One died, three were kept prisoner down in the cellar. And, unbelievably, the others were adopted by the father and lived openly upstairs as his 'grandchildren'. No wonder the family's story stunned the world. And I must add a warning - what follows contains strong adult themes.
PETER HARVEY: In the very early hours of the morning, about a month ago, an ambulance drove up this road bringing a seriously ill young woman to the small hospital in Amstetten, a town not too far from Vienna. She looked like a ghost. She was seriously ill. Her only relative, a mysterious grandfather.
PETRA STUIBER: First of all, they didn't know what to do with her so they said, "We have to find the mother very urgently."
PETER HARVEY: Locating the mother had become absolutely essential and after a few days a woman named Elizabeth came forward, and slowly revealed details of a story so horrifying it would shock the world. What Elizabeth Fritzl revealed was that her father Josef had kept her locked in a cellar for 24 years and here she gave birth to seven of his children, including the daughter now on life support in hospital.
PETRA STUIBER: I thought, "I cannot believe it, this cannot be true! "This can't be true." It is a monstrous case because it is the own father who did this to his daughter. To his own flesh and blood.
PETER HARVEY: And this is where that monstrous crime happened - 40 Ybbestrasse, Amstetten. A place that now ranks with the world's worst crime scenes. It comes as a real surprise to find this house of horrors is on a very busy, very normal suburban street. Shoppers and neighbours absolutely everywhere and yet for 24 years, no-one suspected the appalling crimes that were going on in there.
PETRA STUIBER: This is a 'normal' family and the neighbours were saying, "Oh such a nice man, such a nice woman, "and they were so nice with their grandchildren." And then you find out something like that.
PETER HARVEY: Austrian journalist and commentator Petra Stuiber has been following the case closely and she is ashamed it could happen in her country.
PETRA STUIBER: I am very angry. I am angry with it because I cannot believe that anyone can do this to a woman. And I think this says a lot about our society because no-one really wanted to go deeper and to look into the case and to ask some questions.
PETER HARVEY: Certainly no-one questioned Josef Fritzl. An electrical engineer, he and his wife Rosmarie were well-respected in this town. In truth, Fritzl was a convicted rapist who had spent time in jail. He was a cunning, meticulous man who planned his crime years in advance.
(TRANSLATION) FRANZ POLZER: We now believe he planned his own personal empire as early as 1978, and to start a relationship with his pretty daughter Elizabeth, in the cellar.
PETER HARVEY: Chief Investigator Franz Polzer is in charge of this case. And with his help we can show you for the first time Fritzl's hidden hell-hole. The cellar is reached through one, two, three... ..eight locked doors in all. To the final hidden entrance to Elizabeth's prison. Beyond that, Fritzl had purpose-built a prison with a kitchen, a bathroom a living area and beyond that, two bedrooms. It was cramped, dark and airless.
(TRANSLATION) FRANZ POLZER: I went to see this dungeon, this prison, for myself once. I went through it and I was very glad to be able to leave. The environment in this room where the ceilings were very low - around six-foot at the highest point - the environment was anything but pleasant, because every-day living, personal hygiene and so on, must have kept the level of humidity high.
PETER HARVEY: Josef Fritzl and Rosemarie had seven children - five girls and two boys. Elizabeth was the fourth child, and one day back in 1984 she simply disappeared. Or, at least, that's what her father led everyone to believe.
PETRA STUIBER: Well, he was very persuasive, and he he convinced everyone that his daughter did run away. Because she was very difficult and even if she was a child she was a difficult child.
PETER HARVEY: For the first four years Elizabeth was alone, except when her father came down and raped her. Over the next 20 years she had seven of his children, all born in the cellar. Michael died soon after birth and was incinerated by Fritzl at the house. Bizarrely, of the six remaining children Three were chosen to live upstairs. The others - Felix, now 5, Stefan 18 and Kerstin 19 - were condemned to life in the dungeon. And it was down here, that Elizabeth tried to give them as normal a life as possible.
DR CHRISTOPH HERBST: They are well-raised, very educated, very polite, so... ..that's really very amazing.
PETER HARVEY: Dr Christoph Herbst, Elizabeth Fritzl's lawyer, was stunned by how well she and the children coped. Reading, writing...
DR CHRISTOPH HERBST: Reading, writing, mathematics and these things. She had books, she asked her father to bring her books and some learning materials and then she tried to educate her children. I think they had two or three hours per day and they just had to learn something.
PETER HARVEY: Josef Fritzl has never explained why he chose just three of the children to live upstairs. Each was taken from Elizabeth shortly after birth and placed on the front doorstep. Fritzl told his wife that their runaway daughter had simply dumped them in the dead of night, with a note. Wouldn't you think that with all of this going on, that Rosemarie, the mother, would start asking questions?
PETRA STUIBER: Maybe she feared about the answers.
PETER HARVEY: Do you think she knew?
PETRA STUIBER: We still do not know what his wife really knew.
PETER HARVEY: She took in three babies - one after another - and accepted them. What do you think?
PETRA STUIBER: She was so suppressed that she didn't raise questions she didn't dare to raise questions. She accepted everything, for years and years.
PETER HARVEY: With his upstairs family totally submissive and his secret downstairs family locked away, a super-confident Fritzl went on a string of holidays like this sleazy sex tour of Thailand. Who looked after the family in the basement? Who looked after the prisoners downstairs in the basement? How did he get away with that?
PETRA STUIBER: Nobody looked after them. Because he was he was a very good logistic thinker and he planned everything. He had some rooms where they could store milk and bread and all the things you need, even vegetables. And yes, they were fine for three weeks, that was OK.
PETER HARVEY: There was one thing that Fritzl couldn't control - when one of the kids in the dungeon got seriously ill, he had to do something. And that brings us back to this hospital where things started to unravel. It was a few weeks ago. The eldest of the cellar kids, 19-year-old Kerstin, became seriously ill and remarkably, Fritzl agreed to call an ambulance. Her first time, ever, outside. Doctors were immediately suspicious. The young girl was as white as a sheet from severe vitamin-D deficiency - a total lack of sunlight. Police were baffled. Who was she? Where was her mother? Back in the basement, Elizabeth finally stood up to her father, demanding to go and see Kerstin and again, remarkably, he agreed. What do you think Elizabeth said to Fritzl? I mean, what do you think she said to convince him?
PETRA STUIBER: I think she said, "You will be guilty of murder," or something like that. And she must have been outrageous and in a very precarious state of mind...
PETER HARVEY: To see her daughter so sick?
PETRA STUIBER: I think he feared that everything could explode downstairs.
PETER HARVEY: In fact, the entire situation was so explosive that Josef Fritzl knew he had to move very fast to prevent 25 years of lies and deception from blowing up in his face. So, astonishingly, he went down to the dungeon basement and brought up Elizabeth and the two remaining imprisoned children to meet the family upstairs. And the way he sold it? Elizabeth had come home because she was so concerned about her daughter in hospital. At the hospital, police began questioning Elizabeth. At first they suspected her of being an abusive mother. But finally, 24 years of horror came bursting out.
PETRA STUIBER: First of all they didn't believe her because it was such an amazing story and it was like a really bad thriller, or something like that.
PETER HARVEY: Because she must have said to them, "He has kept me in a dungeon..."
PETRA STUIBER: For 24 years. He has raped me several times, I bore him seven children, one died after birth.
PETER HARVEY: And the young policemen or women who heard this, what do you think they said to themselves?
PETRA STUIBER: I think they needed psychological help themselves. Because you cannot bear something like that. You cannot listen to something like that and not be touched in a very, very deep way.
PETER HARVEY: What motivates a man like this?
DR PAUL BRITTON: Most men who fall into this pattern of offending have, I think, two things, They have a corrupted lust, and a desire, an urge, for possessive control.
PETER HARVEY: Dr Paul Britton, one of Europe's leading forensic psychologists, believes Fritzl planned the imprisonment of his daughter when she was very young.
DR PAUL BRITTON: Men who offend against their children, their daughters, they don't begin when she a young woman of 18, they begin when she is a child. They blame her for arousing in them illicit feelings. So what they are able to do - they are able to push away from themselves responsibility and they put it onto someone else But what you really have is a straightforward, predatory lust.
PETER HARVEY: For the survivors of 40 Ybbestrasse the sad irony is that after all those years locked in the cellar they are now locked away in a psychiatric hospital under heavy guard. Elizabeth's lawyer, Dr Herbst, had just visited when we spoke to him. She is 42, she's been living in a dungeon since the age of 18, how does she look now?
DR CHRISTOPH HERBST: She looks like a normal woman. She has a very, very white face, absolutely, very pallid face, of course, because she hasn't had sunshine or something like that but she looks completely normal. Just as you would expect it from someone else who is 42-years-old.
PETER HARVEY: Dr Herbst says it is heart-breaking to see them still confined indoors. The kids can't wait to get out. They haven't walked in the rain, they haven't stood in the sun, they haven't swum in the river.
DR CHRISTOPH HERBST: The small boy said, "I've never experienced rain on my skin "so I would like to experience that." He didn't see thunder storms.
PETER HARVEY: They've never walked on the grass, and they've never smelt flowers?
DR CHRISTOPH HERBST: Never. Never.
PETER HARVEY: From his jail cell Josef Fritzl, at 73 years old, is said to be so deluded that he can't see what he has done is wrong. It is not known yet whether he will end up in a normal prison or a psychiatric ward. Either way, it is now this monster who is facing the rest of his days locked away. He was an evil man, wasn't he.
DR PAUL BRITTON: Most people who I've come across in these circumstances at one point or another would wish to argue, "I'm not well, I must be mentally ill." But at the bottom of it, there is a perverse sexual drive and the intention, the need is to bend everyone else as an object - not as a person, but as an object to use in this world and so, they are bad.