Mr Fritzl confessed to imprisoning his daughter for 24 years
Austrian investigators are examining a police file on "house of horrors" father Josef Fritzl for a rape in the city of Linz dating back to 1967.
The file has been handed to prosecutors in St Poelten, a local paper, Oberoesterreichische Nachrichten (OOeN), reported on Friday.
Normally it would be locked away for 50 years under Austrian law.
Mr Fritzl, 73, is being questioned by police over his 24-year imprisonment and rape of his daughter in a cellar.
Austrian officials in Amstetten, where Mr Fritzl lived, said they did not know he had any previous convictions when they were conducting background checks on him.
Josef Fritzl is being questioned by police in St Poelten
Under Austrian law convictions can be expunged after as little as five years.
Mr Fritzl was allowed to adopt three children, out of seven he had secretly fathered by his daughter Elisabeth.
The woman who claims she was raped by Mr Fritzl in 1967 told OOeN this week: "As soon as I saw his picture on TV I knew: It was him."
The paper reports that Linz police had also recorded Mr Fritzl as a suspect in two other sexual offences.
Seven children were born from the abuse in Amstetten, three of whom remained incarcerated with her, never seeing daylight until they were released earlier this week.
Elisabeth and the children are now in care with the Austrian authorities, who are protecting their privacy at a psychiatric clinic. The oldest daughter, Kerstin, is fighting for her life in hospital.
Mr Fritzl is refusing to answer any more questions, as police try to piece together his life.