Mr Fritzl confessed to imprisoning his daughter for 24 years
A lodger at the Austrian house where a father allegedly imprisoned and abused his daughter says he saw another man go to the cellar where the abuse happened.
Alfred Dubanovsky told the BBC the man was introduced as a plumber.
His claim contradicts those of investigators who say the father, Josef Fritzl, had no accomplice.
Police say Mr Fritzl's daughter, Elisabeth, was imprisoned and sexually abused by her father in the cellar for 24 years, in the town of Amstetten.
Seven children were born from the abuse, three of whom remained incarcerated with her, never seeing daylight until they were released earlier this week.
Mr Dubanovsky, rented a room in the Fritzl house for 12 years.
He and other lodgers were forbidden to go down to the cellar under threat of eviction, he told the BBC.
Fritzl's lodger tells his story
In his ground floor room he heard noises coming from the cellar, but Mr Fritzl passed it off as the gas heating system.
Mr Dubanovsky assumed the basement was being used as a storeroom because a neighbour said Mr Fritzl often took food down to the cellar.
However, officials believe that no-one was aware of the existence of the purpose-built dungeon, citing DNA tests.
"I think we can rule out accomplices," Leopold Etz, chief of homicide investigations for Lower Austria province, told the Associated Press.
Meanwhile, a sister-in-law, Christine R, told the Oesterreich newspaper that Mr Fritzl used to go into the cellar every morning at 0900 "apparently to draw plans for machines, which he sold to firms" .
"Often he even stayed down there for the night," she added. "Rosi [his wife] wasn't allowed to bring him a coffee".
The newspaper did not give Christine's family name.
She said Mr Fritzl "was a despot, I hated him". She said he "always belittled" his wife Rosemarie, who has told police she knew nothing about the captives in the windowless cellar.
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, in police custody, is refusing to answer any more questions, as police try to piece together his life.
Investigators are examining the cellar door, to see how Mr Fritzl operated it. He told police that he used a coded keypad to open it remotely.
Police are checking his claim that the heavy reinforced concrete door would open automatically if he were absent for a long time.
He also reportedly told his victims they would be gassed if anything happened to him. Technicians are trying to establish if this was more than a threat.
Former Austrian kidnap victim Natascha Kampusch told the BBC that Elisabeth Fritzl and her family would need "a lot of silence" to recover, adding: "Time heals all wounds."
Ms Kampusch was kidnapped aged 10 and held in a basement cell for eight years, until she escaped in August 2006.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.