A wealthy Russian could have died from eating seafood during a romantic meal with his lover or been the victim of a deliberate poisoning, an inquest heard.
Experts refused to dismiss the possibility a "malignant" force was to blame, and it was too late to test for nerve agents such as Novichok, the Old Bailey heard.
Alexander Perepilichnyy, 44, collapsed while out jogging near his home in Weybridge, Surrey, in November 2012.
The married father had spent the night before with his ex-model girlfriend Elmira Medynska, 28, at the Buddha Bar in Paris.
The court has heard he sent back "bad" tempura prawn and ate either sushi or sashimi then vomited repeatedly when he got back to his five-star hotel.
Coroner Nicholas Hilliard QC is examining how Mr Perepilichnyy died, whether he was poisoned and who might have had a motive for murder.
Dr Peter Wilmshurst said fish poisoning could result from eating salmon, tuna and mackerel and cause vomiting, rashes and cramps.
The cardiologist, who had suffered fish poisoning himself, said: "It's rarely fatal. There are cases of people who have died of it.
"It can do all sorts of things to the heart. It often causes the heart rate to go fast.
"If one accepts he had scombroid fish poisoning that night then dies the next day having had a condition 18 hours earlier, if you cannot find any other reason, that becomes the number one suspect."
He added: "The big problem is there are so many unknowns."
Dr Paul Rice said some chemical and biological agents could not be dismissed because the window for testing had closed.
They included cyanide and nerve agents such as VX and Novichok, the court heard.
But Dr Rice said exposure to nerve agents was "unlikely", adding: "Mr Perepilichnyy did not display symptoms. I stand by that."
Professor Robin Ferner said Mr Perepilichnyy may have suffered some sort of food poisoning but cast doubt on it being the cause of death.
He told the court that while tests for poisons had been "extensive" they had not been "exhaustive".
He listed various delayed action poisons such as an ancient gout medicine colchicine, which causes vomiting but was not identified in the body.
MAOI (monoamine oxidase inhibitor) can be sprinkled on a meal and reacts with dark chocolate, which Mr Perepilichnyy was particularly fond of, the court heard.
Fiona Barton QC, for Surrey Police, said: "You have posited a number of possibilities that arise from the evidence that is available and none of those possibilities are in your view likely."
Prof Ferner said: "That's true. The question is not whether they are likely, it's whether they are more likely than an independent cardiac cause."
He refused to say if a heart problem was "more likely than not" the cause of death, and there was laughter in court as he added: "If I could, that would be helpful to proceedings."
Dr Geoffrey Kite, from Kew, first raised the possibility that an unknown compound in Mr Perepilichnyy's body could be the poisonous plant gelsemium - otherwise known as heartbreak grass.
But testing ruled it out "beyond reasonable doubt", he said.
John Beggs QC, for the family, suggested that Mr Perepilichnyy was used to "fine dining" and would know if food was "not right".
He pointed out that he did not "make love" with the former model on the eve of his death, which would be consistent with him having fish poisoning.
The court heard a Viagra-like substance, which was found in Mr Perepilichnyy's body, would not have affected the heart.
The inquest was adjourned until Friday.