DNA tests prove that Austrian Josef Fritzl, who kept his daughter her prisoner in a cellar for 24 years, was the father of her six surviving children, police said today.
Fritzl has confessed to imprisoning his daughter Elisabeth in the cellar beneath their two-storey home and fathering seven children by her.
Prosecutors said they were investigating Fritzl over the death of the seventh child and that he could face a charge of killing the child through neglect.
"The result...shows that the six children, which the unfortunate Elisabeth Fritzl gave birth to in the basement, have all been undoubtedly fathered by her own father, the now 73-year-old Josef Fritzl," Franz Polzer, head of the criminal investigation unit in Lower Austria, told a news conference.
Police have said Fritzl has admitted to burning the body of the seventh child when it died soon after birth.
"Josef F. is being investigated for murder by failing to render assistance," local chief public prosecutor Peter Ficenc told Reuters.
Investigations were also being conducted for rape, incest and coercion, Ficenc said.
Investigators were still searching the 60 square metre cellar beneath electrical engineer Fritzl's home, Franz Prucher, head of security in Lower Austria said.
They have also checked other properties owned by Fritzl, and have cleared them.
"Down there it is just chaos at the moment. We have to go over every detail very carefully," Prucher told Reuters.
Fritzl appeared before a judge in St Poelten, the provincial capital of Lower Austria, on Tuesday, who ordered that police could keep him in detention while inquiries continue.
Officials said Fritzl said nothing on the advice of his lawyer. He was calm when he arrived yesterday and had been put in a cell where he can be monitored in case he tries to commit suicide, said Guenther Moerwald, head of St Poelten prison.
Elisabeth Fritzl, 42, says her father lured her into the cellar of their home in 1984 and drugged and handcuffed her before imprisoning her.
Three of her children, aged 19, 18 and 5, had been locked in the cellar with her since birth and had never seen sunlight. The younger two were boys, the eldest a girl.
Three other children - two girls and one boy - were adopted and brought up by Fritzl and his wife Rosemarie.
The case unfolded when the 19-year-old daughter became ill and was taken to hospital. Doctors appealed for her mother to come forward to give details of her medical history.
The doctor treating the young woman, Albert Reiter, said on Tuesday her condition was critical and her artificially induced coma would continue for several more days.
"Our patient is in a severely life-threatening condition which resulted from a lack of oxygen caused sometime between Wednesday and Friday when she was admitted," Reiter told German broadcaster N24.
The case has shocked Austria, less than two years after an Austrian teenager, Natascha Kampusch, escaped from the basement where she had been locked up by an abductor for eight years.
"There are a million unanswered questions," investigator Polzer told Reuters. "How could he manage to live with what he had done? How did he fool everyone?" Polzer said.
He said he did not blame authorities for missing the case. "I have not been made aware of any error on their part."
"Fritzl was a very cunning man. He not only fooled his wife, but officials, the police, everyone."
Fritzl brought Elisabeth and her remaining two children out of the cellar after the young woman was hospitalised, telling his wife their "missing" daughter had chosen to return home.
Fritzl kept his daughter and three of the children in a complex which was in some places no more than 1.7 metres (5 ft 6 in) high and contained a padded cell, according to authorities.
Photographs of the cellar show a narrow passage leading to rooms that included a cooking area, with children's drawings on the walls, a sleeping area and a small bathroom with a shower.
Fritzl had hidden the entrance to the cell behind shelves and only he knew the code for the concrete door.
Authorities have been asking how events in the house, in a busy street with shops in the small industrial town of Amstetten, 80 miles west of Vienna, went unnoticed.
Commentator Petra Stuiber wrote in Austrian daily Der Standard that what she termed a rich, self-satisfied society needed to examine why it was allowed to occur.
"How is it possible that nobody heard or saw anything? How can it be that nobody asked questions?" said Stuiber.