( CNN ) -- Austrian investigators Monday released more details about the elaborate underground cellar where Josef Fritzl kept his daughter imprisoned for 24 years , along with three of their children . Josef Fritzl admitted to authorities he raped his daughter and fathered her children . Investigators believe Fritzl planned to build the cellar as early as 1978 , shortly after , according to his daughter , he began raping her at age 11 or 12 , said police spokesman Franz Polzer . The 73-year-old Austrian began building the dungeon as part of an addition to his home that year , and simply added the hidden space -- which was not recorded in any building plans -- Polzer said . It took Fritzl until 1983 to finish the addition , Polzer said . Investigators recently discovered another door to the dungeon prison , which was blocked by a 500-kilogram ( 1,100-pound ) steel and concrete door that Fritzl probably stopped using when he later constructed an electronic door for a second entrance , Polzer said . Fritzl , who police believe was the only one with access to the cellar , had to travel through an elaborate maze to get to the prison . `` You would have to open up a total of eight doors , and ... ( for the ) last door which would go into this space ( where the family was imprisoned ) , you would also have to use electronic opening apparatus , '' Polzer said . `` We will have to find out perhaps later from now if perhaps there are other spaces we have n't discovered yet , and perhaps maybe there is something else interesting . '' Fritzl was recently arrested and confessed to holding his daughter , Elisabeth , captive in the dungeon under the Fritzl home for decades , repeatedly raping her and fathering seven children -- six of whom survived . Three of the children were adopted by Josef Fritzl and his wife after he concocted the ruse that Elisabeth had left the babies on their doorstep . The story of the family 's imprisonment began to unravel more than two weeks ago , when one of the children still in the dungeon , 19-year-old Kerstin Fritzl , fell seriously ill with convulsions . The father agreed to take her to a hospital , the first time she was allowed out of the prison where she had spent her entire life with her mother and two brothers . Dr. Albert Reiter , who is treating Kerstin , said Monday that while her condition is still `` grave , '' it `` has improved somewhat . '' `` She has become more stable , but despite that we have to continue to keep her under sedation and give her respiratory help , '' Reiter said , noting it is not clear how long she will be kept under sedation . Elisabeth and her two sons were reunited with her mother , Rosemarie , who police say knew nothing about the basement prison . They were also reunited with the three children that Josef had taken from Elisabeth . The reunited family is living in secluded quarters at a psychiatric clinic , where they are finding a daily routine and adjusting to sunlight -- something the two boys had never seen -- according to the clinic 's chief doctor . `` The mother and the smallest child have , in just the last couple of days , increased their sensitivity to light , '' Dr. Berthold Kepplinger said . `` So we have been able to equip them with protective sunglasses . '' Five-year-old Felix is `` getting more and more lively , '' Kepplinger said . `` He 's fascinated by everything that he sees around him -- the fresh air , the light , and the food -- all of these things are helping them , '' he said . `` Slowly the color of their skin is changing back to a more normal ( shade ) . '' He also said the family members are still getting to know each other and live together as a family . Kepplinger praised Elisabeth for having provided a daily living routine for her children during their captivity . He said the family is getting into a new routine in which the mother and the grandmother make breakfast for the family , and the children make their beds . However , he said there is a noticeable difference between the pace of life of the children held in captivity and that of those who grew up in Fritzl 's home . He said the mother , Elisabeth , takes breaks and naps several times a day . The health of the family members is satisfactory and hospital staff have been able to let more and more light into the rooms where the family is staying , Kepplinger said . Kepplinger said the children , after being confined to a small space their entire lives , are finding it increasingly easy to be in larger spaces . Initially the dungeon where Fritzl held his daughter was only 35 square meters . In 1993 , around the time Elisabeth was pregnant with her fourth child , Fritzl decided to add to the dungeon , building another room that increased the entire living space of the family to about 55 square meters . On Wednesday or Thursday , prosecution authorities will attempt to question Fritzl -- who is no longer talking to police following his initial confession , state prosecutor Gerhard Sedlacek said . A warden at the St. Poelten jail , where Fritzl is being held , told CNN that Fritzl appears to be doing well , but he is refusing to go on walks outside the building where he is detained . E-mail to a friend .
NEW : Hospitalized incest daughter 's condition is grave but stable , police say . Fritzl imprisoned and raped daughter , also fathered her children , police say . Wife of Josef Fritzl was too scared to question him , her sister says . Score: 6