Police are investigating whether the Austrian Josef Fritzl was involved in the murder more than 20 years ago of a young woman who bore an “incredible” similarity to the daughter he was already holding captive in his own cellar.

Mr Fritzl, 73, 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.

Officers revealed today that they were searching for a possible connection between Mr Fritzl 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.

Commentary: a society that nurtures secrets

What society would accept the probability of three children being left on the doorstep of Josef F and grant him adoption?

Multimedia

• Pictures: house of horror

Background

• Why are child abuse scandals happening in Austria?

• Analysis: damage caused may never be healed

• Austria: sedate town is stunned by news

• Police search other Fritzl properties in Austria

Related Links

• Austria fights back against 'Land of Dungeons' tag

• Kampusch - Fritzls should stay in cellar

• Fritzl's victims: the scale of the damage emerges

Police at the time believed that the murder had a sexual background, as the girl appeared to have suffered a sexual assault before she was murdered.

Inspector Alois Lissl of the Upper Austria police said that the investigation was made more difficult by the fact that sophisticated DNA collection techniques had not been developed at the time of the crime, making it hard to cross-check any evidence.

He told Times Online that a witness statement obtained this morning suggested that Ms Posch could have been picked up in a car by an unknown person, but that person appeared to be younger than Mr Fritzl.

"We are investigating the possible connection, but at this stage the probability is not very large," he added.

Mr Fritzl has been remanded in custody and faces various charges, including a possible manslaughter charge over the death of one of the seven children born to Elisabeth during her incarceration.

It is not yet clear who tipped the police off about the possibility of Mr Fritzl’s involvement in the Posch killing but Inspector Lissl said that the victim bore a remarkable resemblance to Elisabeth. “We have compared the two pictures and the similarity with the daughter of Josef Fritzl was incredible,” he said.

Investigators are now probing Mr Fritzl's properties - he owns at least five houses in and around Amstetten - for any evidence of his possible involvement in the murder of Ms Posch, in particular her handbag, blue denim jacked, and a pair of black ankle boots.

The handbag is believed to have contained her personal ID and a set of keys and police sources said that her killer could have kept the missing items as a kind of tropy.

Inspector Lissl played down that theory. "From a criminalistic point of view, I would presume that these items have been long destroyed," he said.

Meanwhile, Mr Fritzl's lawyer has said that there was no evidence that the allegations to back the allegations of rape or kidnapping made against his client.

Rudolf Mayer, a Vienna lawyer, has also said that his client will not say “another word” to the police as he has “said enough already”, implying that Fritzl will only speak to authorities though his legal representatives.

Mr Mayer said that “the case must be resolved”, but claimed that the “allegations” of “rape and kidnapping” were by no means supported by evidence. He added that at the next court hearing on May 13, he would ask authorities to consider whether it was legally justified to keep Mr Fritzl in custody.

Mr Fritzl was “shattered and an emotionally broken” man, Mr Mayer said, adding that he only accepted the case because he believed that people had “good souls”.

Mr Mayer said: “Every case that has got a psychological background is interesting. Us defence lawyers believe that there are good souls.”

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