Listening devices had been placed in the handset of 64-year-old Michele Mongelli's telephone as part of the probe against him and his son Giuseppe, 41.
Officers heard the scrap metal dealer carry out the attack on his daughter, 34, after the handset had been misplaced. One investigator involved in the case said: "It was horrific."
Meanwhile specially trained psychologists were questioning the woman and her four nieces as they pieced together a shocking case of child sex abuse said to have been carried out by a father on his own daughter for 25 years.
Mr Mongelli is said to have raped his daughter and "groomed" his son Giuseppe to abuse not only his sister but also his own four children, aged between six and 20.
The two men were arrested last week in what has already been dubbed Italy's version of the Josef Fritzl case - the Austrian who was jailed for life earlier this month for enslaving and raping his daughter for 24 years and fathering her seven children.
There was shock in Turin that such a horrific case had gone unnoticed for so long - especially as the eldest victim, named as "Laura", had escaped from the room where she was being held in 1994 and told police she was being abused.
But officials dismissed her claims after psychologists concluded she was "disturbed" and Mr Mongelli, who is from Turin, also managed to persuade them that his daughters' claims were fiction.
The woman has given two detailed statements to police and new details of her horrific ordeal in a "darkened room with no electricity" have emerged.
In one statement she said: "From when I was nine years old my father sexually abused me, he touched my intimate parts and he would kiss me on the mouth.
"Then from when I was 16 he forced me to have full sex with him. I would sleep i the same room as him because he wanted to have me near him all the time, next to his bed."
In one chilling transcript released by police the eight year old girl was heard saying: "Dad - get your hands off me. You are a bastard, stop it."
As part of the probe officers were also investigating claims that Mongelli had also sexually abused his brother's daughter during the 1960s.
Pietro Forno, the prosecutor leading the case, said: "The victims were not only subjected to sexual abuse but also psychological abuse.
"I have dealt with many incest cases but this is the worst that I have seen because it lasted for 25 years and also because the main victim is 34 years old.
"I don't want to blame the psychologists who were relied on in 1994. but it's clear that the signs were there and not acted upon at the time when clearly they should have been. However when acted upon again the interceptions and secret recordings proved vital in this case and they make for shocking listening as the abuse can quite clearly be heard."
In a bizarre move Mr Mongelli's family invited the media into their home as they defended him. His other daughter said: "My father never laid a finger on any of us. I kiss the ground my father walks on."
Two of his sons interviewed on Italian television also dismissed the allegations. One said: "Everything that is being said about our father is a lie."
Another showed reporters a tattoo of his father and said: "I have always kept my father in my heart and here on my arm."
Mr Mongelli's wife, Caterina, said:"None of this is true. It's all lies. He is a good man. My husband in in jail for something he didn't do. He kept her inside because he was worried something might happen. He was only looking after his children - what father would not do the same?"
Antonio Genovese, the man's lawyer, said:"My client is incredulous and is trying to find some sort of explanation for these allegations which he denies completely.
"He is ill and a diabetic - after he was arrested he had to be taken to the Molinette Hospital in Turin for checks. I will be appealing immediately for his release from custody."
Lead prosecutor Pietro Forno, 64, is one of Italy's top child sex investigators and worked in Milan for more than 20 years before moving to Turin. He has one of the highest conviction rates of Italian prosecutors and secured the country's first computer child sex pornography case conviction.
Last night Giuseppe Mongelli's lawyer Antonio Foti, said: ''My client denies these allegations completely.
''These accusations come from a woman who is unbalanced, confused and disturbed.
''With regard to his own children he has not laid a finger on them and the eldest one who has provided the details is doing this as part of an act of rebellion against her father.
''These transcripts that have been released or not evidence of sexual abuse they are merely evidence of a cultural and social degradation that existed within the family.
''I have already applied for bail and I have also requested an independent psychiatric reports on the man's daughter and the four children.''
Source ; http://www.telegraph.co.uk/news/newstopics/joseffritzl/5066402/Italian-Fritzl-Police-listened-in-as-suspect-raped-his-daughter.html
