Austrian police are investigating whether Josef Fritzl was involved in another unresolved murder from the 1960s when a teenager was killed in what was suspected to be a sexually-motivated crime.

Officers are now looking into a possible connection between Fritzl, 73, and the murder of Anna Neumayr, a 17-year-old girl who was found dead in a cornfield near her home in Pfaffstaett bei Mattinghofen in Lower Austria in 1966, according to reports.

Ms Neumayr was killed with a captive bolt pistol used to slaughter live stock and police at the time suspected a sexual motive.

A police spokesman said: “We will investigate whether there is anything to indicate that Josef Fritzl was near the scene at the time.”

Less than a year after the murder, in 1967, Mr Fritzl, then 32, was convicted for a rape and an attempted rape and served 18 months in prison. He was also arrested for indecent exposure in public.

Mr Fritzl has already confessed to having fathered seven children by his daughter, Elisabeth, now 42, after locking her in a home-made dungeon under the family home in Amstetten in 1984.

He is also facing manslaughter charges as one of the children produced from the sexual assaults on his daughter died shortly after birth, and Mr Fritzl hurled his body into a heating oven.

This is the second possible murder link that Mr Fritzl is being investigated for, after officers revealed today that they were searching for a possible connection between him and the murder of Martina Posch, 17, who was found near the shore of a lake in western Austria two years later.

Ms Posch disappeared from her home on November 12, 1986, and her tied-up body was found ten days later on the Mond Lake, near the spot where Rosemarie Fritzl, 68, Josef's wife, had a campsite and guesthouse.

Meanwhile, Elisabeth Fritzl’s first boyfriend has spoken for the first time about the time they spend together immediately before she was locked up by her father.

The man, named only as Andreas K, 42, went to a cookery school with Elisabeth in 1984, where she trained to become a waitress, while he was a cook apprentice.

They both spent two months at a boarding school for the duration of the courses. The couple saw each other only one month before Mr Fritzl locked her up in his cellar and kept her as a sex slave for the next 24 years, fathering seven children with her.

Mr K believed the story that she had joined a cult and was shocked to learn the truth about her decade-long ordeal from the media.

Mr K described Sissy, as he called Elisabeth, who was 18 at the time, as a “pretty, but serious and withdrawn girl.” He said: “I saw her in the school yard for the first time. I quickly fell in love with her and I noticed that she was slowly opening up and started to show interest in me.

“It was not so simple to be intimate because such things were not allowed in the school and there were only few opportunities to make out. The girl’s dormitory was a strict taboo and any boy who would be caught there would have been expelled from the school.”

According to Mr K, they used to go for long walks in the woods, where they would spend time together. Although they were physically intimate, the couple never had sex because Elisabeth “would suddenly pull back”.

At that stage she had already been sexually abused by her father for over seven years, according to what she told police.

“She spoke of her parents and her home only once, and said that she had a very strict father. She said he got her a waitress apprenticeship at a tank station, but that she would have preferred to become a cosmetician.”

The last time he saw Elisabeth was when her parents picked her up from the school to take her home in July 1984, only one month before her ordeal begun. Mr K was not allowed to say goodbye as the girl climbed into her father’s grey Mercedes because she was banned from talking to boys.

A month later, on August 28, Josef Fritzl lured Elisabeth into a dungeon in his cellar and locked her up for the next two and a half decades.

Mr K said: “As we kissed goodbye, we promised each other to write as often as possible. But than I thought she had lost interest in me. Now I realise she was no longer able to answer my letters.”

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