Italian scientists believe they have uncovered a 420-year-old murder.

Historians have long suspected that Francesco de' Medici, Grand Duke of Tuscany, and his second wife Bianca Cappello did not die of malaria but were poisoned — probably by Francesco's brother, Cardinal Ferdinando de' Medici, who was vying for the title.

Now, forensic and toxicology experts at the University of Florence report evidence of arsenic poisoning in a new study published in the British Medical Journal.

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As rulers, art connoisseurs and financiers of kings, the Medici family flourished for centuries in the rough-and-tumble alliances of old Europe, providing four popes and ruling first Florence, then all of Tuscany, from about 1430 to 1737.

Its most famous members include Lorenzo de' Medici, or Lorenzo the Magnificent, who supported Michelangelo, Leonardo da Vinci and Sandro Botticelli.

At least two Medici women — Catherine and Maria, who was Francesco's daughter — married kings of France, and the Medicis' former home, the Pitti Palace, now houses an important art gallery.

Among nobles, deaths by poisoning were common, and deadly concoctions of the day had such deceptively pretty names as "Secreta Secretissima," and "La Cantrella."

"When a Medici dies, the first assumption is arsenic," said Richard J. Hamilton, a medical toxicologist at Drexel University who was not involved in the study.

Francesco de' Medici ruled from 1574 until his death at age 46 on Oct. 17, 1587, 11 days after he fell ill and a few hours before his wife, who by all accounts had been his mistress while he was married to his first wife — who is also believed to have died of poisoning.

"Immediately after their deaths, rumors began to circulate that they had been poisoned," Donatella Lippi, a professor of history of medicine and a co-author of the study, told The Associated Press. It "was a lethal dose, but progressive, and the symptoms were compatible with arsenic poisoning."

From the outset, Ferdinando de' Medici's behavior was suspicious and fueled rumors, the study says. Among other things, he took charge of his brother's illness, compiling medical bulletins and minimizing the gravity of his brother's condition in dispatches to the Vatican.

After the deaths, he ordered immediate autopsies — an unusual step, apparently taken to protect himself from future accusations.

More importantly, the symptoms reported by doctors treating Francesco — nausea, violent vomiting, cold sweats, gastric burning — were typical of arsenic poisoning and not of malaria, the study says.

"There was always a suspicion [of murder], but there wasn't scientific proof," said Marcello Fantoni, who teaches Renaissance history at Kent State University.

Lippi and three other scientists — Francesco Mari, Aldo Polettini and Elisabetta Bertol — tested a fragment of femur and beard hairs with skin tissue still attached taken from Francesco's tomb in the Medici Chapels in Florence. Bianca's grave was never found.

They also tested organ remains found in broken terra-cotta jars buried under the crypt in the Church of Santa Maria a Bonistallo, near Francesco's villa.

Lippi said DNA tests showed it was "highly probable" that one set belonged to Francesco. The other, which also showed evidence of arsenic poisoning, was female, Lippi said.

"They were inside a hole," she said. "There was a bunch of dirt and then I found two metal crucifixes, then pieces of what seemed to me like soft tissue."

Tests showed the beard hair had a low concentration of arsenic inconsistent with chronic exposure. However, tests on liver samples taken from the jars showed acute arsenic poisoning, the researchers said.

Angelo Moretto, a clinical and experimental toxicologist with the International Center for Pesticides in Milan, said the study was solid, though he would have been more cautious given the low concentrations of arsenic in the beard hairs.

"They make accusations that are quite strong. I would have been more low key about it," he said.

According to the study, the concentrations of arsenic are consistent with the fact that the couple survived for 11 days.

"These important findings, in addition to the historical data collected on the events before and after the almost simultaneous deaths of the grand ducal couple, allow us to rewrite the historical reconstruction of those events and to affirm that acute poisoning with arsenic was the cause of death of Francesco I de' Medici and Bianca Cappello," the study says.

"It sounds pretty reasonable," said Hamilton. "They've done an efficient job of matching the DNA and they've done a good job of establishing that the arsenic was not contaminated."

The only surprise, he said, is that Francesco could have been poisoned so easily.

"He was a notorious poisoner, and the symptoms were classic, and Francesco would have been quick to recognize them," Hamilton said.

As for who did it, "I think it was Ferdinando," Lippi said. "He was the one who benefited. He was eliminating his brother, and he eliminated the hated Bianca."