Investigators said Monday they believe Josef Fritzl began plotting as far back as 1978 — when his daughter was 12 — to build the windowless warren of rooms where he imprisoned her and fathered seven children with her.

Officials said Fritzl secured the underground cell with eight doors, sophisticated locks and electronics. The main door weighed 500 kilos (half a ton), part of Fritzl's elaborate plan to keep intruders out and his captives in.

Police Col. Franz Polzer told reporters that by 1978 local building authorities approved plans for an expansion of the apartment building Fritzl owned in Amstetten, 75 miles (120 kilometers) west of Vienna — and that Fritzl had ideas that went far beyond the blueprints.

"We are working with certainty on the idea that already in the planning phase he had the intention to build a small space, a small secret, a small dungeon unknown to the building authorities," Polzer said.

"We can't just look back to 1984," the year Fritzl allegedly imprisoned his daughter Elisabeth, Polzer said.

"We have to go back a few years earlier, to when construction apparently took place," he said. "The logic says the idea was already there, or an obsessive thought played a role, to build this jail, dig out these rooms and later to equip them and imprison the daughter there."

• Click here to see photos of Josef Fritzl and the "House of Horrors."

Although the heavy door guarded the main entry, investigators uncovered a second entry involving multiple doors, including one made of steel and protected by an electronic code, he added. He said the underground area was enlarged after Elisabeth had her fourth child.

Police claim Fritzl, 73, has confessed to holding Elisabeth and raping her for 24 years. He has not yet been charged and remains in pretrial detention. Prosecutors said they planned to meet with him for the first time this Wednesday or Thursday.

Fritzl's lawyer, Rudolf Mayer, indicated he is preparing an insanity defense.

In a broadcast interview, Mayer said he believes Fritzl has a serious mental disorder and that anyone with that kind of psychological illness "didn't choose" to do what police allege he did.

Mayer said experts will have to determine Fritzl's mental state and decide whether the suspect can be considered certifiably insane. If that is the case, and Fritzl is convicted, he would be confined to a psychiatric institution rather than a prison, he said.

"I believe that the trigger was a mental disorder, because I can't imagine that someone has sex with his own daughter without having a mental disorder," Mayer said. Elisabeth, now 42, was 18 when she was imprisoned.

Berthold Kepplinger, director of the psychiatric clinic that has been counseling and caring for Elisabeth, Fritzl's wife and his illicit children said the victims are slowly learning how to live as a family.

He said the children were given a fish tank and the youngest — a 5-year-old boy — received a teddy bear.

"Both sides of the family are slowly growing together," Kepplinger said. "What is nice is to see how the family is starting to organize daily life. Mother and grandmother are preparing breakfast and dinner together. The children are making their beds themselves."

Authorities first began to unravel the complex story on April 19, when a 19-year-old girl who Fritzl fathered with his daughter was admitted to a hospital suffering from a lung infection.

Doctors, unable to find any medical records for the girl, appealed on television for her mother to come forward. Fritzl then accompanied Elisabeth to the hospital on April 26 and opened up to police.

The 19-year-old remained hospitalized Monday in critical but stable condition, although clinic spokesman Klaus Schwertner said her situation "has stabilized somewhat in recent days." Officials said she is still being kept in an artificial coma to help her breathe.

Investigators have said they believe Fritzl concealed his crimes from his wife, and Polzer on Monday reiterated officials' belief that the retired electrician acted alone.