London: Josef Frizl, who raped his daughter up to 3,000 times over a 24 year period after locking her up in a dungeon, planned to kill his cellar family by dipping them in an acid bath, said the Austrian's prison cell mate.

Fritzl told his cell mate, 19-year-old Patrick Jakob, that he was tired of living two lives and wanted to rest in his old age. Jakob spent three months locked up with Fritzl in St Poelten jail, Austria.

"He (Fritzl) said he had two choices - to let them all go and face justice, or kill them.

"He said he had considered how he would kill them and the best option would be an acid bath, so all traces of his secret family would be destroyed," The Sun qouted Jakob as saying.

"That way no one would ever know what he had done. He (Fritzl) could make them all disappear. He had all the details worked out."

Fritzl wanted to kill his daughter Elisabeth, now 44, and three of the seven children he fathered from her in the dungeon.

Fritzl locked Elisabeth in the dungeon basement at his home in Amstetten when she was 18, and told his wife Rosemarie that she had run away.

One of the seven babies he fathered with her died and he burned the body. Three others were locked up with Elisabeth. The other three were raised by Rosemarie and Fritzl. He told his wife that he found them abandoned.

Jakob said: "He (Fritzl) said he put Elisabeth in the cellar for her own protection, because he thought she was taking drugs.

"I was young and naive and I fell for his arguments at first. But with hindsight I could tell he was just manipulating me."

Fritzl was arrested in April 2008 and is likely to spend the rest of his life in a psychiatric institution.

"He never got to make the decision whether to free his cellar family or to kill them. But I have no doubt he would have done, given the choice. He is that evil. He tried to make out he was a good man because he hadn't killed his family and they had been set free. What nonsense," Jakob said.

Recalling the days he spent with Frizl, Jakob said: "When I saw my new cellmate was Fritzl, I was scared, genuinely terrified. I'm not proud of my crime, but it was a teenage mistake. How could I be in the same cell as a monster?"

Fritzl told Jakob that his crime of burning down two farm houses wasn't so serious.

"I was stunned at how friendly he was. You would never know what he had done.

"There are two Fritzls - the upstairs Fritzl, the one who lived a normal life, and the downstairs Fritzl, who raped his daughter in the basement.

"Most of the time I saw the upstairs Fritzl, but over time I saw his other side. If he didn't get to see the news at 5 p.m., he could turn nasty."

The former prisoner said that Frizl used to get dozens of letters from fans.

"One day he showed me a photo sent to him. There was a woman standing next to a masked man. She was laughing. It was macabre. The fans saw him as the chief monster. They respected him."

Jakob said that Frizl sobbed after listening to Elisabeth's testimony in court

"He was broken. He knew it was all over. He lay down on his bed and sobbed uncontrollably. He looked a pathetic old man.

"On his last day in St Poelten he rose at 7 a.m., packed his gear and left. We didn't say goodbye. I hope I never see him again."