Josef Fritzl, the Austrian who admits imprisoning his daughter and fathering children by her, may seek to avoid jail by pleading insanity, his lawyer indicated in comments published on Sunday.

"In my personal opinion, Josef Fritzl is mentally ill and therefore of diminished responsibility," lawyer Rudolf Mayer told German weekly Bild am Sonntag in an interview.

"I believe that my client does not belong in prison but in a secure psychiatric unit."

Until now Mayer has given no indication of how Fritzl would plead in any trial, saying he did not believe his client would receive a fair hearing in view of the huge amount of press coverage the case has attracted.

If he believes that a psychiatric evaluation of Fritzl (73) ordered by the court does not give a proper assessment of his client he will consider ordering a separate report himself, Mayer told Bild.

He added that he was "not defending a monster but a human being, even if that is hard to take for some people. I am already receiving threatening letters saying that I belong in the cellar with Mr. Fritzl."

Fritzl, currently in custody, has admitted imprisoning his daughter in a specially made dungeon in his basement for nearly a quarter century where he fathered seven children by her, one of whom died, according to Austrian police.

Mayer's comments came as Franz Polzer, heading the police investigation in the town of Amstetten, said police were close to completing their examination of the bunker.

"Our puzzle is almost complete," Polzer told the Austrian newspaper Kurier, adding that there was no evidence that Fritzl had any accomplices.

A major task is to establish whether Fritzl was serious about his alleged threat to gas the occupants of his windowless bunker if anything happened to him.

Polzer confirmed to the Austrian news agency APA details about the dungeon from Fritzl's daughter Elisabeth's published by German weekly Der Spiegel.

The magazine said that for the first nine years of her imprisonment the dungeon comprised just one room, implying that incestuous sex or rape took place in the presence of young children.

From 1984 to 1993, "repeated rapes committed by Josef Fritzl" took place in front of children born in 1988, 1990 and 1992, the report quoted Elisabeth's testimony to police as saying.

It also said Elisabeth was handcuffed to a post for the first two days underground, when she was 19 years old. She was then tied to a leash for the next six or nine months so that she could go to the toilet on her own.

Polzer also told Kurier that Fritzl had already installed a thick steel door in his cellar before imprisoning his daughter there in 1984.

The British mass-circulation newspaper the Sunday Mirror published an interview with Polzer, quoting him as saying Fritzl had raped his daughter in front of the children.

"She remained in one room for nine years, until Fritzl started expanding the dungeon with extra rooms in 1993," the Mirror report said.

"But even this didn't stop him dragging Elisabeth in front of her children and raping her before their eyes," it continued.

Fritzl had also installed punishment cells, Franz Polzer was quoted as saying.

"Rooms in the main part of the cellar contained odd bits of furniture or toys rescued by Elisabeth for her children. But the punishment cells were utterly bare," said the Mirror report.

"They are distinctive and stand out as they look just like a prison cell," it quoted Polzer as saying.

Areas of the dungeon appear to have been under construction and it was possible Fritzl might have been planning to expand it even further.

"But we simply don't know what his intentions were," the Mirror also quoted the officer as saying: "He is no longer co-operating with us, which makes it hard, but he has admitted all the charges to us."

AFP