The Austrian man who held his daughter captive in a cellar for 24 years and fathered seven children by her was convicted of rape around the time the daughter was born, his sister-in-law told Austrian media.

The newspaper Oberoesterreichische Nachrichten yesterday reproduced what it said was a 1967 court record from state archives in the city of Linz, in which a Josef F. was accused of breaking into the apartment of a 24-year-old nurse and raping her.

Josef Fritzl's sister-in-law, identified as Christine R., told the daily Oesterreich that he had gone to jail for the offence. "I was 16 when he was locked up for rape," she said.

In a video interview aired by a number of news channels, she said she believed Mr Fritzl had spent a year and a half in jail, and that his wife Rosemarie, despite her shock, had tried to hold the family together.

Austrian officials will say only that, if Mr Fritzl had such a rape conviction, it would have been purged from the records after 15 years at the latest. A call to Mr Fritzl's lawyer, Rudolf Mayer, went unanswered yesterday.

The reported conviction dates from shortly after the birth of Mr Fritzl's daughter Elisabeth, now 42, who was released just over a week ago from the cellar where he had fathered seven children with her.

The head of the investigation, Franz Polzer, said in an interview that Mr Fritzl was a classic tyrant personality.

"This man, all of whose crimes were driven by his sexual energy, never once tolerated being asked about his holidays, his absences," he said.

Authorities have said officials followed correct procedures in allowing Mr Fritzl and his wife, who had seven grown-up children of their own, to care for three of the children he had with his daughter, ostensibly after she abandoned them on their doorstep.

Elisabeth was kept imprisoned in a cellar complex beneath the grey apartment block with her three other surviving children - a daughter, now 19, and two sons aged 18 and five. Until their release the previous weekend, those children had never seen sunlight.

Detectives working in the bunker are able to work for only an hour at a time because of the foul air. They say they are looking at the electrical systems and mechanics of the dungeon, including the hidden, reinforced electric door that Mr Fritzl used to seal from the house above.

Mr Polzer confirmed a report in the German magazine Der Spiegel that Elisabeth had been restrained on a leash for the early months of her captivity.