Faced with tough questions about how the shocking crimes could have gone undetected for so long, chief investigator Franz Polzer said on Wednesday there was absolutely no indication Josef Fritzl, 73, was working with anyone.

Fritzl has admitted to keeping his daughter locked in a windowless bunker and repeatedly sexually assaulting her. Yet despite reports he was previously been convicted of attempted rape, the authorities saw no reason to investigate him further when he applied to adopt three of those children.

The head of Amstetten social services, Hans-Heinz Lenze, told a news conference that by that time, any convictions had been expunged from his record as laid down in Austrian law.

"At the time of the first adoption on May 20, 1994, there were no convictions on either his record or that of his wife," Lenze said, presenting a copy to reporters with the permission of the man's lawyers.

Police said they were probing a possible link between Fritzl and the unsolved murder of a teenager in 1986 although they admitted there was no direct connection so far.