AMSTETTEN, Austria (CNN) -- Three children freed from a cellar in which their mother had been imprisoned and raped by her own father for 24 years had never seen daylight, police in Austria have confirmed.

A hidden, narrow bedroom found in the cellar of the house where Josef Fritzl held his daughter captive. more photos »

Police spokesman Franz Polzer told CNN that 73-year-old Josef Fritzl admitted holding his daughter, Elisabeth Fritzl, 42, hostage in the windowless cell and fathering seven children by her.

"The mother had memories [of the outside world] and got used to the situation," Polzer told a press conference Monday afternoon. "The others knew nothing else."

The main question reverberating from the small Austrian town: How could a man keep his daughter locked in his basement for 24 years, where she gave birth to seven of his children while her mother and three of those children lived upstairs without an inkling of the horrors in the cellar?

Fritzl explained Elisabeth's disappearance by saying she had run away from home, a story backed up by letters he forced Elisabeth to write, including one that begged her parents not to look for her.

Other letters made it seem the missing daughter had left the three children on the parents' doorstep -- when in fact they had been born in captivity in the family's basement.

Elisabeth told police that she and her three children Kerstin, 19; Stefan, 18; and Felix, 5, did not see the light of day during their entire time in captivity underneath the building in Amstetten, a rural town about 150 km (93 miles) west of Vienna.

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Elisabeth is described as "very disturbed" and having trouble talking to police about her ordeal, reports CNN correspondent Fred Pleitgen. She went missing in 1984, when she was 18 years old, police have said.

More details also emerged at the news conference about the basement dungeon in which the daughter and her children were kept -- and how her father managed to keep them captive for more than two decades. See inside the cellar prison »

The authorities have revealed that the prison, constructed in the basement of the 1960s building, ran underneath both the building itself and the garden outside.

The entrance was via a small door, hidden behind cupboards in the basement, controlled by an electronic keyless-entry system. Polzer said that the prison was hard to find, even if someone was looking for it, and had been soundproofed.

"Even though they shouted and called they were not in a position to let anyone hear them," Polzer told the press conference.

Polzer said that Fritzl made clear to his wife and other children that the area was out of bounds and they were not to go into the basement. He bought food and took it to his captives in the evening. Watch a report on details of the case »

Detectives made the grim discovery about the cellar earlier this month after Kerstin was hospitalized in Amstetten after falling unconscious and taken to a hospital in Amstetten by her grandfather with a SOS note from her mother hidden on her.

A DNA test was later carried out which revealed her grandfather, Josef Fritzl, was also her father, according to ORF, Austria's state-run news agency.

That sparked a police investigation, which revealed that Fritzl fathered at least six children with his daughter, forcing her and three of the surviving children to live in the cellar of his house, according to ORF's Peter Schmitzberger. Watch police describe the captives' jail »

On Sunday, police searched the hidden rooms where Fritzl admitted he kept his daughter and their children, including sleeping quarters, a kitchen and a bathroom, which Fritzl told police he built, Polzer said.

Amstetten police say they were put on Fritzl's trail following an anonymous tip off. They apprehended the pair on Saturday near the hospital and once police assured the daughter that she would never have contact with her father again, "she was able to tell the whole story," Schmitzberger said.

Elisabeth said her father began sexually abusing her at age 11. On August 8, 1984 -- weeks before she was reported missing -- her father enticed her into the basement, where he drugged her, put her in handcuffs and locked her in a room, she told police.

For the next 24 years, she was constantly raped by her father, resulting in the six surviving children, she said, according to the police statement.

She also told police she gave birth to twins in 1996, but one of the babies died a few days later as a result of neglect, and Fritzl removed the infant's body and burned it in an oven.

She told police that only her father supplied her and her children with food and clothing, and that she did not think his wife knew anything about their situation

Fritzl lived upstairs with his wife, Rosemarie, who police said had no idea about her husband's other family living in the cellar. The couple adopted three of the children that Fritzl had with his daughter, according to police. He told his wife that his missing daughter had dropped the unwanted children off at the house because she could not take care of them, police said.

When Kerstin fell ill, Fritzl apparently told his wife and the hospital that his "missing" daughter had dropped off the sick girl on his doorstep.

In an effort to find out about Kerstin's condition, the hospital and police asked the media to put out a bulletin requesting any information about the girl or her missing mother, attorney general Gerhard Sedlacek told NTV.

Sometime later, Fritzl brought Elisabeth out of the cellar, telling his wife that she had returned home with her two children after a 24-year absence, police said.

He took Elisabeth to the hospital to talk with doctors about Kerstin's condition, and at that point, authorities became aware of her situation, Sedlacek said. E-mail to a friend

CNN's Fred Pleitgen, Ben Brumfield and Nadine Schmidt contributed to this report

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