He told doctors that the 19-year-old had been dumped on his doorstep by his long-lost daughter Elisabeth, who left a note asking for help.

As the doctors could not determine the cause of the teenager's illness, they asked for her mother to come forward to provide a medical history.

That was when Fritzl allowed his daughter and her two other children aged 18 and five to leave their underground home for the first time.

He made up a story that his missing daughter had finally chosen to return home.

According to ORF, Austria's state-run news agency, a DNA test later revealed that Fritzl was not just Kerstin's grandfather but also her father.

On April 26, police, acting on a tip-off from an unidentified person, detained Fritzl and Elisabeth near the hospital for questioning.

Police yesterday said that the 73-year-old had confessed to holding his daughter captive for 24 years and fathering seven children by her.

To his neighbours in the industrial town of Amstetten with a population of 22,000, Fritzl was a likeable and polite man who was always ready to help others.

No one could begin to imagine that he was in fact leading a horrific double life.

'He managed to create a legend, one which everyone believed,' said Interior Minister Guenther Platter.

Apart from Elisabeth, now 42, Fritzl and his wife Rosemarie have six other children.

During questioning and assured by the police that she would have no further contact with her father, Elisabeth gave details of her ordeal which began in 1984 when she was 18.

That was when, she said, her father lured her into the basement of the block where they lived and drugged and handcuffed her before imprisoning her.

She told police she had been raped repeatedly by her father from the age of 11.

To cover up his daughter's disappearance, Fritzl told his wife that she had run away to join a cult and did not want to be found.

During her captivity, Elisabeth gave birth to seven children. One of them, a twin baby born in 1996, died a few days later. Her father took it away and burned it.

The two oldest children - Kerstin and Stefan, 18 - and five-year-old Felix had been locked up since birth and had never seen sunlight.

As for the other children - Lisa, 16, Monika, 14, and Alexander, 12 - Fritzl raised them after telling his unsuspecting wife that Elisabeth had abandoned them.

Each time, he forced his daughter to write a letter, such as the one in 1993: 'The baby is nine months old. She will have a better life with grandma and grandpa than with me.'

Investigators described the basement prison as a sophisticated network of chambers with facilities for sleeping, cooking and washing. An electrical engineering technician by training, Fritzl entered the basement through a hidden door operated by a secret code, police said.

'There was a shelf with plenty of cans and containers, and behind the shelf was a door made of reinforced concrete, secured electronically and running on steel rails, and only the suspect knew the code,' said local official Heinz Lenze.

Newspaper headlines called the case the 'crime of a monster' and stories questioned the authorities and residents of Amstetten for failing to notice 'the martyrdom in the horror house'.

The children and Elisabeth, who is described as frail and deeply disturbed, are receiving psychological counselling.

The case is all the more shocking because it reminded many people of one involving another Austrian girl. Natascha Kampusch was 10 when she was abducted by by Wolfgang Priklopil in 1998 and locked up in a cell for eight years before escaping in August 2006.

'The community of Amstetten, including its population, should drown in shame...just like in Strasshofen with Priklopil,' the Oesterreich newspaper wrote in an editorial.

REUTERS, AGENCE FRANCE-PRESSE