LONDON — The bright yellow trumpet-shaped flowers of Gelsemium elegans are only found among the foothills and mountains of Asia. Beautiful to look at, they trigger a rapid assault on the nerve and respiratory systems when swallowed.

The plant’s deadly qualities were first identified in the 1870s by Sir Arthur Conan Doyle, the creator of Sherlock Holmes. Despite warnings that the Kremlin wanted Alexander Perepilichnyy dead, modern British detectives concluded that there was nothing suspicious in his sudden collapse during a run in the suburbs outside London in November 2012.

Despite warnings that the Kremlin wanted the 44-year-old whistleblower dead, British detectives eventually concluded that Perepilichnyy’s death was not suspicious.

That was before Professor Monique Simmonds, a leading botanist at Kew Gardens in West London, stunned British officials by disclosing that traces of Gelsemium elegans were present in Perepilichnyy’s body. The plant, which is also known as “heartbreak grass,” had not been identified in any of the original toxicology investigations.

“Given that it is a known weapon of assassination by Chinese and Russian contract killers, why was it in his stomach?” asked Bob Moxon Browne, a lawyer speaking at a pre-inquest hearing in Surrey on Monday. The coroner halted the inquest and ordered further tests to determine whether the plant’s poisonous properties had spread through the Russian’s body. The plant was reportedly used in the murder of a Chinese tycoon in 2012, when a business rival sprinkled the poisonous herb into a cat meat stew.

Claims that another Russian dissident has been assassinated on British soil will further damage relations between London and Moscow. The alleged murder of dissident Alexander Litvenenko with a radioactive pot of tea has led to the greatest period of hostility since the Cold War.

Perepilichnyy was said to be one of the most crucial witnesses in a $200 million fraud case against Russian officials, police officers and the Russian mafia. American executive Bill Browder, head of the investment firm Hermitage Capital, employed lawyer Sergei Magnitsky to investigate how the Russian officials had targeted his company. Magnitsky, 37, was arrested in Russia and subsequently died in police custody.

The lawyer’s death was condemned by governments all over the world. President Obama signed the Magnitsky Act, which placed sanctions on a number of Russian officials involved in his detention, in December 2012.

A few days before President Obama signed the act, Perepilichnyy had set out for his usual run across the St George’s Hill estate, a gated community home to Kate Winslett, Ringo Starr, and Elton John. He never returned.

He’d had been given a clean bill of health during a medical assessment for a life insurance policy shortly before he died. So, his sudden collapse did strike detectives suspicious but crime scene investigators and pathologists could find no sign of foul play.

Perepilichnyy had approached Hermitage with emails that allegedly incriminated some of the Russian tax officials. It was said to be the breakthrough in the investigation into one of the biggest tax frauds in Russian history. He was expected to testify in a Swiss court in a case that would have embarrassed Moscow and suggested there was endemic corruption in Russia.

In an interview with The Guardian earlier this year, Browder, the head of Hermitage, said he had written to Surrey police immediately to say he had been receiving death threats. “I’m certain he was murdered. The police kicked it into the long grass. This is a travesty of justice,” he said.

Browder said death threats for those involved in the case were common. Another of his colleagues received a text message alluding to Michael Corleone’s famous line in The Godfather Part II: “If history has taught us anything, it is that you can kill anyone.”