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."
"SELF-SATISFIED SOCIETY"
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, 130 km (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.
