The lawyer for the Austrian man who allegedly imprisoned his daughter for 24 years and fathered seven children with her has said he is preparing an insanity defense.

In an interview broadcast late Sunday, attorney Rudolf Mayer said he believes Josef 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.

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

Investigators have said Fritzl, 73, confessed last week that he held his daughter captive in a windowless cell, fathered her seven children, and tossed the body of one who died in infancy into a furnace.

"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.

Fritzl will make his first appearance before prosecutors Monday, and police planned to brief reporters on the status of their investigation into a case that has stunned Austria and the world.

Fritzl has not yet been charged, but remains in pretrial detention. His victims are receiving psychiatric care and counseling.

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

Investigators have said they believe Fritzl concealed his crimes from his wife, Rosemarie, and her sister said Rosemarie believed her husband's cover story that Elisabeth had run away from home to join a cult.

"We were all taken in by him," the sister, who gave her name only as Christine R., said in an exclusive television interview with The Associated Press. "Every person that looked in his eyes was fooled by him."

Christine R. described Fritzl as a "tyrant" who instilled a culture of fear at home, which may have explained why his wife and other children apparently never dared venture into the cellar — which Fritzl warned was strictly off-limits.

"When he said it was black, it was black, even when it was 10 times white," said the woman, who was interviewed Saturday evening at her home in Austria. "He tolerated no dissent."

"Listen, if I myself was scared of him at a family party, and I did not feel confident to say anything in any form that could possibly offend him, then you can imagine how it must have been for a woman who spent so many years with him," she said.

She also said her imprisoned niece — now 42 — ran away from home as a 17-year-old, about six months before police say she was locked into the secret basement rooms. The previous attempt to flee may have made her father's story that the girl joined a cult more believable.

She also said Fritzl was jailed for "a year and half" for an alleged 1967 rape conviction, but said she could not offer details.

On Saturday, the Oberoesterreichische Nachrichten daily printed an excerpt of what it said was a 1967 court record found in the state archives in Linz, in which a Josef F. was accused of breaking into the apartment of a 24-year-old nurse and raping her.

Police have declined to comment, saying records that old would have been erased under Austria's statutes of limitation. But authorities are awaiting old court records that the media say document the case.