"It was astonishing how easily it happened - how the mother and grandmother came together," clinic director Berthold Kepplinger told reporters Tuesday.
Kepplinger said the family members interacted very naturally - even though the three children who lived upstairs with the grandparents had never met their siblings in the windowless cell.
Officials said one of the children who is receiving medical treatment at another hospital was not part of the gathering.
Word of the reunion came as police announced that DNA tests confirmed Fritzl is the biological father of the six surviving children he had with his daughter.
Police also said they combed through Fritzl's other properties but found no other hidden windowless cells like the one where he had held his daughter - now 42 - captive since she was 18.
Police said Fritzl confessed Monday to holding the daughter captive, sexually abusing her, fathering her children and tossing the body of one child who died in infancy into a furnace.
Officials had said Fritzl faces up to 15 years in prison if charged, tried and convicted on rape charges, the most grave of his alleged offenses under Austrian law.
But prosecutors in Lower Austria said Tuesday they were looking into the possibility of charging Fritzl with "murder through failure to act" in connection with the infant death. Murder in Austria is punishable by up to 20 years in prison.
Investigators say they believe his wife, with whom he had seven children, was unaware that the daughter she believed ran away to join a religious cult in 1984 was living as a prisoner beneath her feet.
Fritzl's lawyer, Rudolf Mayer, said his client also was under psychiatric care and "is really hit by this. He is very serious, but he is emotionally broken."
But prosecutor Gerhard Sedlacek said Fritzl was "completely calm, completely without emotion" when he was formally placed in pretrial detention Tuesday.
Austria is still scandalized by a 2006 case involving a girl who was kidnapped and imprisoned in a basement outside Vienna for more than eight years, and residents of Amstetten were puzzled as to how the Fritzl case could go undetected for so long.
"How is it possible that no one knew anything for 24 years?" asked Anita Fabian, a teacher in Amstetten. "This was not possible without accomplices."
Regarding the apartment building that Fritzl owned and lived in, the town's authorities authorized the construction of an addition with a basement in 1978, city spokesman Hermann Gruber told the Austria Press Agency. He said inspectors examined the project in 1983 - the year before the young woman went missing - and nothing looked suspicious.
Police said the surviving children are three boys and three girls, ranging in age from 5 to 19. Officials said three of the secret children - aged 19, 18 and 5 - "never saw sunlight" until they were freed a few days ago.
Police released several photos showing parts of the cramped basement cell, with a narrow passageway leading to a tiny bedroom. Investigators said an electronic keyless-entry system apparently kept them from escaping.
Three of the children lived with the grandparents. Fritzl and his wife registered those children with authorities, saying that they had found them outside their home in 1993, 1994 and 1997.
Leopold Etz, a regional police official, told APA that Fritzl apparently chose which of the children would live upstairs with him and his wife according to whether they were "crybabies."
Officials said social workers made regular visits to the family but found nothing out of the ordinary, reporting that Fritzl's wife was attentive, the three children were doing well in school and clubs, and that all of them played musical instruments.
The case unfolded after the eldest of the secret children, a 19-year-old woman, was found unconscious and gravely ill on April 19 in the building and was taken to a hospital.
Hospital officials said the 19-year-old remained in critical condition Tuesday because of the effects of lack of oxygen, and was undergoing dialysis.
Amstetten Mayor Herbert Katzengrueber told the AP in an interview that Fritzl was personable and well-liked, and that the town had honored the suspect and his wife in 2006 for their 50th wedding anniversary.
Katzengrueber said he was at a loss to explain how such an atrocity could happen.
"No one can really explain it," he said. "I am appalled and saddened that such a thing could happen in my hometown. ... These have been awful and sad days."
