Josef Fritzl, the Austrian who held his daughter captive for 24 years and fathered her seven children through multiple rape, threatened to kill all of them by turning the sealed underground bunker in which they were held prisoner into a gas chamber, police said yesterday.

The disclosure formed part of a catalogue of disturbing evidence that police in Lower Austria have so far collected on Fritzl, 73, a retired electrical engineer who has now been remanded in custody following the release of DNA tests which prove he was the father of all of his daughter's children.

Fritzl was arrested last Sunday and confessed to police that he had held his own daughter, Elisabeth, prisoner in an elaborately constructed underground bunker beneath his garden in the provincial Austrian town of Amstetten for nearly a quarter of a century.

During that time he beat his daughter, raped her repeatedly and over the years fathered seven children with her. One died as a baby and Fritzl destroyed the corpse by throwing it into his heating furnace. Three of the children were allowed to live with Fritzl and his wife as normal children in their home upstairs. But Elisabeth and her other three children were kept prisoner in the cellar for 24 years until they were found and released by police last weekend.

The Austrian media have since called Fritzl a "Horror Father", but the impression of him emerging yesterday was that of a Jekyll and Hyde figure. He was an obedient small town citizen and family man who joked with neighbours and turned up at work with shining shoes and a fastidiously arranged necktie, but also a domineering, violent and secretive individual who went on sex trips to Thailand and "lost control" of his libido.

Franz Polzer, the Austrian police chief leading the investigation, said Fritzl had given the impression, during protracted interrogations, that after 24 years he now actually believed the web of lies he had constructed to keep his incest a secret from his own family, the police and the public.

"Fritzl insisted that he chose Elisabeth as his favourite daughter, built the bunker just for her and felt that he had to lock her up because he was frightened that she was about to become involved with drugs," Mr Polzer said.

The reality was quite different. When his daughter was 18, Fritzl lured her into the elaborate cellar which he had built under his home during the Cold War in the late 1970s with the help of a government grant available for constructing domestic shelters against nuclear fall-out.

Police said he drugged his daughter with ether and handcuffed her to a wall in the cellar. During the first years he is alleged to have "kept her like an animal". He would go down into the cellar, beat her and then rape her. After two years, Elisabeth gave birth to her oldest daughter, Kerstin. She gave birth to the other children about every two years that followed, apparently without any medical supervision.

Mr Polzer said Fritzl behaved like a military overseer in the cellar and had managed to intimidate his daughter and her children so heavily that they never considered trying to escape. "The door to the cellar is made of thick steel and it has an electric combination lock," Mr Polzer said. " Fritzl told them that if they ever tried anything, he would keep the whole cellar locked up and then fill it with gas pumped in from the outside. Then there would be no escape." Photographs were published yesterday of Fritzl wearing swimming trunks during a sex excursion to Thailand in the early 1990s, leaving his daughter and her three children behind in the cellar. Police said the underground bunker contained a large store room in which enough food could be kept for several days. Fritzl had paying tenants in his home and warned them that they would be kicked out if they ever went near the cellar.

In order to conceal the large amounts of shopping he had to do for his prisoners, Fritzl drove in the evenings to outlying supermarkets to make his purchases. Police said he owned a number of properties in the region apart from his large two-storey Amstetten home and appeared to fund his activities through renting them out. He was also said to have spent a brief period in jail after being convicted of attempted rape in the 1960s. However, under Austrian law, the offence has been erased from police records.

All the time Fritzl maintained the appearance of a normal small town life. Gerda Schmidt, an Amstetten resident who worked with Fritzl at a builders merchant near the city in the late 1980s, said: "All I remember is a very vain man. His shoes were always glistening, his tie was never askew, he could have been a diplomat."

He and his wife formally adopted three of Elisabeth's children after Fritzl forced his daughter to write letters insisting she had given them into their care as she had run away to join a religious sect. Fritzl's wife appears to have believed him.

Amstetten's foster care and adoption authorities said yesterday that they had no reason to doubt Fritzl's story, as he and his wife acted as model parents. Rosmarie Fritzl was said to have regularly attended foster parent meetings and taken her adopted children to school and on outings such as visits to the local fire station. She is on record as having complained that she had no idea where her daughter was.

Karl Heinz Lenze, a local authority official, insisted yesterday that " everything was in order" concerning the Fritzl family and they had seen no cause to investigate further.