AMSTETTEN, Austria — 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, 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.

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 today, 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 family is 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 today 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.