Alexander Perepilichnyy was a Russian expat out on a jog near his mansion in Surrey, England in 2012, when he suddenly collapsed and died. Perepilichnyy, 44, was in robust health but local officials shrugged the incident off as not suspicious.

To Sherlock Holmes, however, this would have been anything but ordinary. Perepilichnyy exhibited signs of deliberate poisoning with the potent Gelsemium elegans, a plant Holmes encountered in 1910’s “The Adventure of the Devil’s Foot” in 1910 (albeit under a different name). The plant is — then as now — favored by Russian and Asian assassins. Add to that the fact that Perepilichnyy was a banker in hiding after exposing the shady tax dealings of the Russian state and Mafia, and the game was afoot.

“That one’s been known as a poison for a long time,” Michael Balick, Ph.D., an expert on poisonous plants and the Vice President for Botanical Science at the New York Botanical Garden, tells Inverse. Arthur Conan Doyle, the real-life physician-author behind Sherlock Holmes, had even tested Gelsemium’s therapeutic uses on himself, publishing his findings in the British Medical Journal in 1879. Self-experimentation aside, Conan Doyle’s interest in the plant wasn’t all that strange: By the mid-1800s, it was already known as a nervous system relaxant, used in folk medicine to treat neuralgia together with illnesses like pneumonia and pleurisy. The failure of Britain’s top police force to identify the toxin seems like a gaffe worthy of Conan Doyle’s bumbling caricature of Scotland Yard. But Dr. Lewis S. Nelson, a toxicologist and co-author of the Handbook of Poisonous and Injurious Plants, warns against being too haughtily Sherlockian in our judgment.

“A bright poisoner would be able to identify it as a plant that could be used to hurt somebody, as many plants are,” he tells Inverse, but he adds that the plant’s active ingredient — an alkaloid known as gelsemine — is only easy to find “if you know what you’re looking for.” The compound kills discreetly, in a mechanism similar to strychnine’s: Both plant-based compounds act on the brain to cause seizures and their accompanying convulsions, later paralyzing the spinal cord and its related organs — including the lungs. The ultimate cause of death is, most often, asphyxiation, though if Perepilichnyy’s case is any indication, it’s not always so easily identified. His official cause of death is still listed as unknown.

It’s likely that the ongoing investigations will lead to official finger pointing, given the especially foreign nature of the substance. The gelsemine purportedly used to kill Perepilichnyy is derived from G. elegans, a plant native to Asia and known as “heartbreak grass” because of its suicide-aiding properties, but other species within the Gelsemium family of shrubs include G. sempervirens and G. rankinii, both of which are native to North America and are occasionally featured in home gardens for their beautiful yellow flowers. It would not have been found naturally in Surrey, where Perepilichnyy was most likely murdered by the mob.

But for every killer compound like gelsemine, there are multiple other toxins that can do the same. As Balick points out, there are an estimated 420,000 species of higher plants on the planet, and thousands containing poisonous alkaloids, some of which could be used for illicit purposes, in the same way as gelsemine. Even Sherlock Holmes couldn’t keep track of all of their effects. But consider that Holmes was also a serious cocaine addict, one that had difficulty drawing the line between what constituted therapeutic use and self-harm. In his view — one that both Balick and Lewis share — all compounds are neutral, and it’s the amount ingested that makes them poisonous. How else would the substance driving Conan Doyle’s investigations into neuralgia treatment also wind up in the corpse of a Russian mafia hit?

Yasmin is a writer and former biologist living in New York. A Toronto girl at heart, her writing also appears in The Last Magazine and SciArt in America. You might recognize her as a past host of Scientific American's YouTube series.