A police inquiry was sparked after a woman claiming to be one of Fritzl's early rape victims spoke for the first time about how she was sexually abused in by the former electrical engineer in 1967, when he was working for a steelworks in the Austrian city of Linz.

The woman, who refused to disclose her identity, told the local Linz newspaper, Upper Austrian News that, in 1967, when she was a recently married young mother, Fritzl had slipped through her ground-floor bedroom window while her husband, an Austrian railway employee, was away.

"I felt the bedclothes being pulled back. At first I thought it was my husband coming home but then I felt this knife being pushed against my throat," she told the newspaper. "He told me: if you make a noise I'll kill you. Then he raped me."

The woman said she had seen Fritzl on a number of occasions in Linz before she was assaulted by him. He had attracted attention because he behaved like a peeping Tom. "He was a voyeur. He used to ride around on his bicycle and watch everyone," she said.

She said that after Fritzl was exposed in the Austrian press and on television as an incestuous rapist, she realised that he was the same man who raped her more than 40 years ago. "I saw his photograph on television and knew it was him by his eyes," she said. Other Austrian newspaper reports confirmed yesterday that Fritzl worked in Linz from 1967 until 1969 as an employee of the Voestalpine steelworks. There have been numerous reports of Fritzl serving a jail term in the late 1960s for rape.

However, the Austrian justice authorities say the offence is irrelevant because it happened more than 15 years ago and has been erased from records.

The police are also reinvestigating the murder of a 17-year-old woman named Martina Posch, whose body was retrieved from Austria's Mondsee lake in November 1986. No one was arrested but subsequent forensic tests indicated she had been sexually abused.

At the time, Fritzl owned a boarding house opposite the spot where the girl's body was found. Police say photographs of Posch show she bears a striking similarity to Fritzl's abused daughter, Elisabeth.