His campaign, which lasted from August 1996 to September 1998, ended when police fitted the pigeons with radio tracking devices and followed them in a helicopter to an allotment in Kronberg, near Frankfurt. There, neighbours told detectives they had seen Nemeth saw up and burn an old pigeon coop. He was later arrested in his flat in nearby Eschborn.
Nemeth told a Frankfurt court on the first day of his trial yesterday how he made the supermarket poison himself, following instructions in a book he had found at a flea market. A total of 25 ready-made food items - including a tube of Nestle mayonnaise in Regensburg, and a tube of Nestle mustard in Saarbrucken - were poisoned, eight with lethal doses of a cyanide compound.
The Frankfurt police investigating the case said at the time they had never dealt with such a serious case of commercial blackmail. Nemeth had demanded that Nestle send him 25m Deutschmarks (more than pounds 9m) in uncut diamonds.
He left a car containing 12 carrier pigeons for detectives in Marburg, near Berlin. Each bird had a small rucksack in which the diamonds were to be placed. But the police fitted them with radio tracers before releasing them.
The carrier pigeon trick was not a new one: two years earlier, police in Lower Saxony had caught another blackmailer who tried to use the same method of getting his money from a company.
Nemeth's confession took the court by surprise. "It was me," he said, after the charges were read out. Until then, he had denied all the accusations. Prosecutors had planned to call more than 80 witnesses and 12 experts to the trial, which was expected to last until November. Officials now have to rearrange the proceedings.
Nemeth could face up to 15 years in prison but a verdict is not expected until some time next month.