Researchers have been trying for years to verify a claim imprinted on an ornately decorated calabash that it contains a sample of the blood of the French king guillotined in Paris on January 21, 1793.
The dried, hollowed squash is adorned with portraits of revolutionary heroes and the text: "On January 21, Maximilien Bourdaloue dipped his handkerchief in the blood of Louis XVI after his decapitation".
He is then believed to have placed the fabric in the gourd, and had it embellished.
The sinister souvenir has been in the private hands of an Italian family for more than a century, said the team of experts from Spain and France which published its findings in the journal Forensic Science International.
Two years ago, analysis of DNA taken from blood traces found inside the ornate vegetable revealed a likely match for someone of Louis' description, including his blue eyes.
But not having the DNA of any kingly relation, researchers could not prove beyond doubt that the blood belonged to Louis.
Using the genetic material, the team managed to draw a link to another gruesome artifact -- a mummified head believed to belong to Louis' 16th century predecessor, Henri IV.
In so doing, they provided evidence for authenticating both sets of remains -- uncovering a rare genetic signature shared by two men separated by seven generations.
"This study shows that (the owners of the remains) share a genetic heritage passed on through the paternal line. They have a direct link to one another through their fathers," French forensic pathologist Philippe Charlier said.
The revolution in which Louis and queen Marie-Antoinette lost their heads in public executions also saw mobs ransack the royal chapel at Saint-Denis, north of Paris -- hauling ancient monarchs like Henri from their tombs and mutilating the remains which they tossed into pits.
An individual was recorded to have rescued a severed head from the chaos.
Long thought to belong to Henri, assassinated at the age of 57 by a Catholic fanatic in 1610, the head changed hands several times over the next two centuries, bought and sold at auction or kept in secretive private collections.
Scientists in 2010 said they found proof that the head was indeed Henri's, citing physical features that matched 16th century portraits of the king, as well as radiocarbon dating, 3D scanning and X-rays.
The 2010 study, however, found no DNA and its findings have been contested by some.
With the new evidence, "it is about 250 times more likely that the (owners of the) head and the blood are paternally related, than unrelated," co-author Carles Lalueza Fox of the Institut de Biologia Evolutiva in Barcelona said by email.
Taken together with all the physical and forensic evidence, historical records and folklore, it would be "extremely surprising" if the remains did not belong to the two assassinated monarchs, he added.
"One can say that there is absolutely no doubt anymore," about the authenticity of the mummified head, added Charlier.
The DNA data obtained from Louis XVI could now be used to decipher the genetic code of France's last absolute monarch and his living relatives.
Discovery News. 2013. “Dried Squash Holds Headless King's Blood”. Discovery News. Posted: January 1, 2013. Available online: http://news.discovery.com/history/dried-squash-holds-headless-kings-blood-130101.html