Police have admitted they have been left at a loss by the attack, which took place nearly a year ago, when the 40-year-old man was attacked by the mysterious stranger in Hannover.
The case mirrors the murder of Bulgarian dissident Georgi Markov who was killed with a poisoned umbrella in London in 1978 at the height of the Cold War.
A post-mortem and toxicological examination being carried out on the German man are expected to point to mercury poisoning as the cause of death, Der Spiegel news magazine said on Friday.
The man was diagnosed with mercury poisoning a few weeks after the attack, and fell into a coma.
He had said he was attacked by a slim stranger who had a sticking plaster on his face.
After months in a coma, the victim had seemed to be improving, a spokeswoman for the state prosecutor said. He had been in a rehabilitation centre, but this Wednesday he suddenly died.
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The prosecutor are now searching for the mysterious assailant, investigating a possible charge of grievous bodily harm resulting in death.