Fritzl, 73, built an extension to his house at number 40 Ybbsstrasse between 1978 and 1983 and 'at least in the planning phase, the intention must have been there to build a secret area, a dungeon', chief investigator Franz Polzer told a news conference here.

Fritzl, currently in custody, has admitted to police that he imprisoned and sexually abused his daughter Elisabeth, now 42, in a bunker beneath the family home since 1984 when she was 18.

Initially, Elisabeth was held in a single room of 35 square metres, but when she became pregnant from the sexual abuse and gave birth to several children, Fritzl extended the dungeon, breaking through walls into other rooms beneath the older part of the house, built in 1890, Mr Polzer said.

By 1993, the dungeon comprised three rooms measuring a total 55 square metres, the investigator said.

Incest suspect refuses to leave cell: prison chief
Fritzl is refusing to leave his cell, authorities said on Monday.

Fritzl has been held at a prison in St. Poelten since admitting last week that he imprisoned and sexually abused his daughter for more than two decades in a windowless bunker beneath the family home.

Prison director Guenther Moerwald told the Austrian news agency APA that Fritzl was an 'unproblematic inmate: he's calm, collected and alert.' However, Fritzl was refusing to take the daily one-hour walk allowed in the prison yard. 'He doesn't want to go out,' Mr Moerwald said.

Fritzl was still in a cell with another inmate.

'It's working very well. Of course, we've talked to his co-cell mate, who's told us he doesn't have any problem' with sharing a cell with Fritzl.

But the suspect was being kept apart from other inmates to prevent any potential violence. 'That's a risk we don't want to take,' Mr Moerwald said.

Fritzl was not being treated differently from other inmates and has access to television, radio and reading materials. 'He takes his meals in the cell. That's normal,' Mr Moerwald added.

A pyschiatrist examined Fritzl on Friday and decided the father did not have suicidal tendencies, the prison chief said.

Fritzl is a typical case of a 'malignant narcissist', Mr Thomas Mueller, one of Austria's best-known criminologists said.

The malignant narcissist could only increase their own self-value by oppressing others. 'They lock them in or inflict pain on them. The perpetrator wants to seen as powerful,' said Mr Mueller, who trained as a profiler with the FBI in the United States.

'To me, it's as if they have a black hole inside them. With every sadistic act, they try to fill that hole. But everytime they do something, the hole just gets bigger,' Mr Mueller said in an interview with Austrian public radio Oe3,

Asked why the criminal therefore persisted with the crime, Mr Mueller explained: 'For a short time, they experience a feeling of relief. They have the feeling they have power over life and death. But then the perpetrator sees that there are some things that didn't fit in with their fantasies. So they seek out another victim.'

'Sexually abusing your own child is about power, because you're exercising control,' Mueller said. 'It's easy to control a young, weak child. But when the child grows up, there's an increased danger that they'll rebel.

'So the perpetrator has to think pragmatically, such as building a bunker' to lock the victim in.' By making their captives totally dependent on them, they feel increased self-worth every time they bring them food, Mr Mueller said. -- AFP