Fritzl's apartment house, its back garden obscured by a tall hedge, was his kingdom, and interlopers were not welcome, one neighbour told the New York Times.

Yesterday, investigators combed the house and garden for clues to how Fritzl, 73, had managed to keep one of his daughters imprisoned for 24 years in a basement dungeon, where she bore him seven children.

The daughter, Elisabeth, now 42, a frail, white-haired 'broken woman', is in psychiatric care, an officer said.

Two of her children are also in psychiatric care. Her eldest daughter, Kerstin, 19, whose illness exposed Fritzl's secret after he had her admitted to a local hospital, is in critical condition.

The authorities said Fritzl confessed on Monday to imprisonment, sexual abuse and incest. A domineering man who brooked no dissent in his household, he faces 15 years' jail if convicted. Yesterday, a court ordered police detention while inquiries continued. He has been put in a cell where he can be monitored in case he tries to commit suicide,

Police said Fritzl first sexually abused Elisabeth when she was 11. When she was 18, he lured her into the cellar, drugged her and handcuffed her there. She was beaten when she tried to protest, and she realised after several years that it was futile to fight off her father.

Colonel Frank Polzer, the head of lower Austria's Criminal Affairs Office, described Fritzl as a man who had been 'extraordinarily sexually potent' and who 'led a perfect double life'.

Trained as an electrical engineer, Fritzl owned the small apartment building, renting out a few apartments and living on the top floor. Over many years, he expanded the dungeon, the entrance of which was a 90cm by 60cm steel door concealed behind a cupboard in an underground workshop. The door could be opened only from the outside or via a remote control Fritzl kept on him. He warned his secret children that only he knew the access code and if they tried to harm him, they would all die in the dungeon.

Inside are five tiny rooms, including a storeroom with rubber-lined walls where one child, who is epileptic, would be taken whenever he had a fit.

Police declined to say if the padded room was used for more sinister purposes, but Col Polzer said the dungeon 'contained a room which made it possible for him to continue his abuse'.

The cellar was originally just 90cm high, said police, but Fritzl, who reportedly served a jail term in the 1960s for a sex offence, added more rooms and installed electricity and water supply, the Times of London said.

He was apparently able to dodge suspicion by buying food and clothing for his prisoners outside town and delivering them at night.

Three of Elisabeth's children - Kerstin, Stefan, 18, and Felix, five - have lived in the cellar all their lives. One child died in infancy and Fritzl raised the three other children - Lisa, 16, Monica, 14, and Alexander, 11 - in his house. The five siblings, Elisabeth and her mother had a tearful reunion on Sunday, the Telegraph has reported.

Child development expert Jay Belsky told the BBC that since the children lived with their mother, it would have cushioned the amount of trauma they suffered.

REUTERS

See also MONSTER DAD OF FRANCE