A Yaroslavl court in June convicted Vyacheslav Solovyov of six counts of murder and sentenced him to life in prison.
According to reports earlier this year on state-run Rossia television, he added poison to the food and drinks of his victims and observed the fatal effects on their health, sometimes over the course of several months.
Solovyov was discovered dead early Tuesday morning in solitary confinement at Detention Facility No. 1 in Yaroslavl, regional Federal Prison Service spokesman Alexei Pichuyev said Wednesday.
There were no indications of suicide or a violent death, and he likely died from the effects of "his experiments with poisons that he gave to others and was exposed to himself," Pichuyev said by telephone from Yaroslavl.
An autopsy was to be performed Wednesday evening, Pichuyev said.
Solovyov began trying to invent poisons to test on humans six years ago, Rossia television reported at the time of the trial in April. Along with six counts of murder, he was also convicted of four attempted poisonings.
Solovyov's first victim was his wife, Olga, whom he poisoned in December 2003 by adding thallium, a highly toxic element used in rat poisons, to her coffee, according to prosecutors. She fell into a coma and subsequently died.
He went on to kill five other people by poisoning them, including his 14-year-old daughter, Anastasia, who accidentally ate thallium-laced caviar he had hidden in the refrigerator, according to prosecutors.
Solovyov continued to poison his daughter, who went in vain from doctor to doctor seeking treatment while her father observed her deteriorating health, Rossia television reported.
She eventually died in August 2004.
In 2006, Solovyov poisoned his girlfriend's mother, who died shortly thereafter. He then poisoned his girlfriend's sister, the sister's husband and their 1-year-old child, prosecutors said.
The child died in March 2007, two days after the first dose, and the parents were placed in intensive care. It was only after the entire family came down with the same symptoms that doctors became interested.
Testing the victims' hair, they found traces of thallium. Traces of the element were found in all of Solovyov's victims as well, prosecutors said.
Solovyov was detained in May 2007 and subsequently confessed to poisoning his victims, Rossia reported.
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