Over the years, Josef Fritzl secretly enlarged the narrow space to add sanitary facilities and small hot plates for cooking. Eventually he gave them a TV set.
Police said that Josef and Rosemarie Fritzl raised three of Elisabeth's children in their apartment. The couple registered those children with authorities, saying they found them outside their home in 1993, 1994 and 1997, at least one with a note from Elisabeth saying she could not care for the child.
Three other children apparently remained in the cellar with Elisabeth and were never allowed to leave the basement.
Elisabeth told investigators she gave birth to twins in 1996, but one died several days later because it was not properly cared for, and her father took the infant's body away to burn it in the furnace, a detail which reportedly was confirmed by Josef Fritzl today.
It was only after Kerstin, the oldest child, was hospitalized that Josef Fritzl freed Elisabeth and the remaining children from their captivity.
He told his wife that Elisabeth and the children had come back to live with them and then took Elisabeth to the hospital so she could discuss her child's medical details with the doctors treating the teenager.
Shortly after the pair had returned to the family home, police officers came to the house to question Elisabeth about neglecting her children. The officers did not take long to persuade Elisabeth to reveal the whole story.
The case is reminiscent of that of Natascha Kampusch, the Austrian girl who was kidnapped when she was 10 years old and held captive in a tiny underground dungeon in her kidnapper's house in a Vienna suburb for more than eight years.
Kampusch escaped her ordeal in 2006, and her kidnapper committed suicide just hours after her dramatic escape.
While police are not connecting the two cases, many Austrians are now asking how such horrific incidents could remain uncovered for so long.
Neighbors in Amstetten told Austrian TV ORF, "They were totally normal, nice next-door neighbors. There was never anything negative about them. How in the world is this possible?"