HORROR FATHER: Fritzl shopping for women's clothes during a trip to Thailand, in a picture taken by his friend. Police said he would buy food and clothing for his captives outside town to avoid arousing suspicions among the residents of Amstetten, hundreds of whom held a candlelight tribute for Elisabeth on Tuesday. -- PHOTO: REUTERS

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Police investigator Franz Polzer said Elisabeth Fritzl wrote to her family who, apart from her father, believed that she had fled to join a cult. She wrote that she wanted to come home but that 'it's not possible yet'.

'He may have had plans to end the captivity at some point,' said Mr Polzer.

'It shows that he must have had a spark of humanity. It also shows how perfectly he planned everything.'

Fritzl had dodged suspicion by buying food and clothing for his prisoners from outside town and delivering them in the night, said the police, who face tough questions about how the crimes could have gone undetected for so long.

They have appealed to about a hundred people who live in the same block as the Fritzls in Amstetten to help piece together details of the incarceration of Elisabeth.

'I ask if perhaps any one of them may have seen something noteworthy that at the time may have seemed insignificant,' Mr Polzer said.

Hundreds in the town held a candlelight tribute for Elisabeth on Tuesday to show their solidarity and outrage.

The appeal came even as police tried to piece together the 73-year-old pensioner's double life as a reputable citizen and a 'horror father' who was 'extraordinarily sexually potent'.

Fritzl had once bought raunchy undies for Elisabeth while holidaying for a month in Thailand, describing her as his 'bit on the side', his best friend Paul Hoerer told The Sun tabloid.

Newspapers said records of a rape conviction in 1960s and another charge of arson and insurance fraud were wiped out after some years as per Austrian law.

'I was 16 when he was jailed for rape. I found that offence repugnant, given how he had had four children with my sister,' Ms Christine R, 56, told the local Oesterreich newspaper.

She also revealed that Fritzl would spend whole nights in the cellar 'developing plans for machines to sell to businesses'.

'He would go down into the cellar every morning at seven,' she said, adding: 'Often, he would spend whole nights down there.'

'Rosi wasn't even allowed to bring him coffee,' she said, referring to her sister, Fritzl's wife Rosamarie.

Police are probing whether Fritzl was responsible for the 1986 unsolved murder of a teenager, although no direct connection has been found so far.

Compiling a complete profile of Fritzl has been difficult because he is refusing to undergo more questioning, said police. Fritzl is receiving special protection in prison as a precaution against attacks by other prisoners.

Meanwhile, two women who have been through a similar ordeal are offering help to the Fritzl family.

Ms Natascha Kampusch, who escaped her tormentor of eight years, advised the family to be careful about going public with their story too soon, if at all. 'They're going to be flooded with different images and impressions. And there are going to be lots of strangers who want to interfere,' she told German TV NDR. She has donated 25,000 euros (S$52,600) to the family.

Another victim, Ms Lydia Gouardo of France, who was tortured and raped by her father for 28 years and has six children by him, has extended a hand of friendship towards Elisabeth. 'I would feel less alone,' she told the daily Le Parisian.

AGENCE FRANCE-PRESSE, REUTERS, ASSOCIATED PRESS