DNA tests today confirmed that Josef Fritzl is the father of his daughter's six surviving children born while she was a prisoner in a cellar for 24 years, investigators said.
Josef Fritzl who has admitted to holding his daughter captive for 24 years and sexual assault.
The 73-year-old could spend the rest of his life in prison if found guilty in a case of incest and sequestration that has left Austrians stunned, prosecutors said.
Earlier, Fritzl had appeared in court to be remanded in custody while doctors shielded 42-year-old daughter Elisabeth and her surviving children in isolation from a world they barely know.
He said little during that hearing, refusing to respond directly to the accusations, said prosecution spokesman Gerhard Sedlacek.
"He risks life imprisonment if manslaughter is established," Mr Sedlacek said.
A second charge of rape and subsequent pregnancy would only bring "five to 15 years in jail," Mr Sedlacek added.
Incest on its own was not punishable with a prison sentence, but holding somebody prisoner against their will could see a sentence of up to 10 years, Mr Sedlacek said, adding: "Under Austrian law, these sentences are not cumulative."
Overall, the trial was expected to last "a not insubstantial period" of several months, the prosecutor added.
Investigators said DNA tests had decisively shown Fritzl was the father of six children born during Elisabeth's incarceration.
A seventh child died shortly after birth and Fritzl confessed to police that he disposed of the body in an incinerator in his building.
It was that point which could see Fritzl charged with manslaughter if prosecutors decide he did nothing to prevent the child's death.
Elisabeth, now 42, and her children now are staying "in a treatment container that can be locked from the inside" to shield them from the outside world, said child and youth pyschologist Paulus Hochgatterer.
Three children never left the three cramped underground rooms where they were held and had never seen natural daylight.
Three other children were legally adopted by Fritzl and lived with him and his wife Rosemarie upstairs in the family home, totally unaware of their siblings imprisoned in the cellar below.
Local authorities said they had made all the necessary background checks for the adoptions.
"As of May 16, 1994 (the date of the first adoption), there is no criminal record for either Josef nor his wife Rosemarie," said the head of the social services in Amstetten, Hans-Heinz Lenze.
According to media reports, Fritzl was previously convicted of attempted rape in the 1960s and of arson.
Social services had made 21 documented calls to the house, as well as undocumented visits, during which Fritzl was usually absent, Mr Lenze said.
Nevertheless, social workers never noticed anything amiss concerning either the children's education or health.
Letters found with the babies explaining that their mother could not take care of them gave the police no reason to search the Fritzl house, Mr Lenze added.
The district court in Amstetten, which must greenlight all adoptions, also insisted it had acted correctly.
"There weren't any doubts about his (Fritzl's) integrity. Why should I put the child in a foster home, when it could grow up in a family," court president Josef Schoegl told the Austria Press Agency.
Safely hidden from the media glare, the treatment of the woman and her children could take several weeks and psychologists were also helping Elisabeth's adult brothers and sisters and others involved in the family, officials said.
"Only very gradually are they being exposed to the outside world," Dr Hochgatterer said, adding that "given the circumstances, they're actually doing quite well."
The two sets of children, who had been completely unaware of each others' existence, were tentatively beginning to get to know one another, even if two of the three who had spent all their lives underground "have a way of communicating that is anything but normal," said Berthold Kepplinger, director of the psychiatric clinic in Amstetten-Mauer.
Doctors would determine when police would be allowed to question Elisabeth Fritzl and the children, but that was unlikely to be for several days.
Authorities were looking for a special school for the children and they have proposed changing the names of Elisabeth, her children and her adult brothers and sisters, said social services chief Lenze.
The case bears striking resemblance to the case of Natascha Kampusch, now 20, who escaped in August 2006 after being kept prisoner for eight years by her tormentor Wolfgang Priklopil.
The Vienna-based psychiatrist who counselled Ms Kampusch at the time, Max Friedrich, said the rehabilitation of the Fritzls would be long and complex.
"You can't lose hope that they'll one day be able to lead autonomous lives, but it'll take years, and scars will remain," for all the children. "Their world has fallen apart," Dr Friedrich said.
The youngest child, five-year-old, seems most able to adapt to his new life and was excited about being able to ride in a car, his carers said.
Chief police investigator Polzer said there was no sign that Fritzl's wife, who also had seven children with him, knew of secret cellar.
"It would go against all logic that a mother of seven children would help the father of those children to look after seven more children whom he had fathered with his own daughter," Mr Polzer said.
The oldest of the children, 19-year-old Kerstin, whose hospitalisation at the weekend led to the discovery of the case, is in critical condition and has been placed in an artificial coma, said the chief doctor of the hospital in Amstetten.
"If there's one thing we've learnt from the case of Natascha, it's that the victims must be given time to rebuild themselves," Dr Friedrich said.