He was hacked to death with an axe by his wife, children and a family friend, his torso later fed to the family's three pet dogs and his head boiled in an urn and buried in a heap of manure.

At least that is the version of events German prosecutors presented to a court in 2005 at the trial of Rudolf Rupp's wife and children, who were convicted over the killing of the family's tyrannical patriarch.

But the case was rolled out again following the discovery by workmen in March 2009 of a Mercedes that had tumbled down an embankment and into the river Danube. Forensic scientists soon discovered that the car was Rupp's and the rotting but intact body found inside was his.

Today a court in Landshut opened the retrial of Hermine Rupp, 55, her children, Manuela and Andrea, and Manuela's boyfriend, identified only as Matthias E, all of whom originally received prison sentences variously for manslaughter and accomplice to manslaughter.

At the heart of the case is the question: how did the prosecutors get their story so wrong? And is it true, as at least one of Rupp's offspring has claimed, that they were threatened with torture if they did not agree to own up to the crime?

The state prosecutor Ralph Reiter faced the somewhat embarrassing task of having to read out the details of the initial and now discredited trial. Rupp was said to have come home one night in 2001 having consumed eight half-litre glasses of beer, to be met in the entrance way by Matthias E who knocked him unconscious with a hammer.

His body was said to have been hacked up and fed to the family's dobermans, a bullterrier and a sheep dog in the farmyard, after which his Mercedes was taken to a scrapyard and destroyed.

No traces of Rupp's blood or bone fragments were ever found in the house or cellar. Nevertheless, prosecutors say they remain convinced that the family and Matthias E were responsible for the farmer's death.

"They were all in on it. They promised themselves they would have a trouble-free life without their father," Reiter told the court.

Lawyers for the defendants say they are pushing for pardons and extensive compensation. Klaus Wittmann, a lawyer for Hermine Rupp, told the court she had given in to pressure on her to confess, blaming "the circumstances of the interrogation, the strain of weeks in custody ... Whatever details didn't fit were accordingly bent into shape."