Josef Fritzl watched car races with his secret family for hours on end on the TV set he provided them in the cellar of their dungeon, according to reports.

The electrical engineer, in prison and under investigation for abducting and sexually abusing his daughter Elisabeth, apparently led a parallel family existence in the cellar beneath his house. It was there that he kept three of his children — a girl, 19, and two boys 18 and 5 — under lock and key with their mother.

Details of police interviews paint a grotesque picture of his attempt to play the role of a halfway normal father. The Austrian magazine News, referring to the interviews, claimed that he would buy the three children toys and play with them while he discussed their upbringing with their mother.

According to News, Mr Fritzl told police that he took only three children to live upstairs with him and his wife, Rosemarie, because she would not have been able to take care of another child at her advanced age.

He is quoted as saying: “Rosemarie would not have managed to take care of another child.”

His mood could change swiftly, according to the reports. On occasions he would beat the children and their mother severely if they did not behave in the way he wanted them. It is assumed that he may have used the padded room in the cellar to rape Elisabeth while the children would be locked up in a different room.

Afterwards he would reportedly then sit on the kitchen table and make his daughter prepare a meal. It was also reported that Elisabeth never told her children that they were in fact imprisoned by their father and did everything possible to create an illusion of normality. She would watch adventure films on TV and then make up stories about princesses and pirates.

Inspector Leopold Etz said that the two boys who spent their whole lives in the cellar spoke their own dialect consisting of gurgling sounds: “To say that the children speak is only a part of the truth. It is a mixture of gurgling sounds and noises. A kind of a dialect of their own.

“But when they want to articulate what they say in order for us to understand them, they do make an effort — presumably out of politeness. But it does cost them a lot of energy.”

Berthold Kepplinger, the head of the clinic where the children are being cared for along with Elisabeth and Rosemarie, said: “They are terribly far from a normal way of expression.”

The regional chief executive, Hans Heinz Lenze, said that in time the children could be relocated and be given new identities to take them out of the public view.

Officers revealed that they were searching for a possible connection between Mr Fritzl and the murder of Martina Posch, 17, who was found near the shore of a lake in western Austria in 1986. Ms Posch disappeared from her home on November 12, 1986, and her body, tied up in plastic bags, was found ten days later on the Mond Lake, near the spot where Rosemarie had a camping place and a B&B.

Police believed at the time that the girl was sexually assaulted before she was murdered. A spokesman said that the investigation was made more difficult because the technique of gathering DNA evidence had not been developed at the time of the crime.

“On the photos taken before her death, the victim appears incredibly similar to Fritzl’s daughter. There is no evidence of any link so far, but we are conducting a thorough investigation to see whether there is any possible link to Josef Fritzl, as it is believed he could have been frequenting the area around the time of the murder,” Alois Lissl, a spokesman, told The Times.

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