By Christian Fraser
BBC News, Rome
Scientists in Italy believe they have uncovered a murder more than 400 years after it is thought to have happened.
The Medicis ruled over Renaissance Florence
Historians had long suspected that Francesco de Medici, the Grand Duke of Tuscany and his second wife, Bianca Cappello, were poisoned.
But for four centuries his death has remained a mystery.
Now, using modern toxicology and forensic equipment, scientists believe they can prove that the Medicis were given a lethal dose of arsenic.
Francesco de Medici was the Grand Duke of Tuscany from 1574 until his death on 11 October, 1587.
He died aged 46, 11 days after first taking to his bed and a few hours after his wife, Bianca Cappello.
It was claimed that the couple had died from malaria but now, after testing remains from Francesco's grave in the Medici chapel in Florence, scientists from the city's university believe they can prove otherwise.
Tests on beard hair proved inconclusive, but scientist Donatella Lippi says samples of Francesco's liver, taken from the crypt, showed levels of arsenic that were significantly higher than those normally found in humans.
"The fact that they survived for 11 days after they became ill was due to the fact that the dose of arsenic was lethal but not high enough to raise too strong a suspicion," she said.
"The description of their death throes included delirious restlessness, and everything matches the hypothesis of poisoning."
But if Francesco was murdered, then who did it? Some believe it could have been his brother, Cardinal Ferdinando de Medici, who was vying for power.
His behaviour at the time of the deaths was said to be suspicious.
He took charge of his brother's illness, compiling the medical bulletins, and after their deaths he ordered immediate autopsies - an unusual step.
But Donatella Lippi says there is another version of the story - that in fact it was Bianca, out to murder Ferdinando, who killed her husband by mistake.
"Bianca had prepared a poison for Ferdinando who did not eat it, while the unsuspecting Francesco helped himself generously," Ms Lippi said.
"According to this version Bianca, in despair, had intentionally followed her husband in his destiny."
The scientists have been testing the exhumed bodies of 21 members of the Medici family to determine how they lived, what they ate, what illnesses they suffered from and how exactly they died.
The most puzzling aspect of this particular case, though, is how Francesco - who had an interest in alchemy and chemistry, and was suspected of poisoning his first wife - could himself have been poisoned so easily and so quickly by someone who was close to him.