It remains unclear how the new toxicology findings will affect the prosecution. The attorney general in Anguilla, Dwight Horsford, has declined to discuss the case, which has moved slowly since Mr. Hapgood’s arrest following the fight on April 13. The defendant was allowed to post a bond of $74,000 and return to his home in Darien, Conn., sparking an outcry on the island that a tourist was receiving preferential treatment.

Mr. Hapgood returned to Anguilla several times last month for preliminary hearings in court that were closed to the public. Those hearings have been adjourned until Nov. 11. Mr. Hapgood, who was traveling with his wife and three children, has been placed on leave by his employer, UBS Investment Bank, pending the outcome of the case.

A GoFundMe page seeking donations to the family raised more than $250,000 before the site pulled it down last week, the Darien Times reported. It is against the site’s policy to allow raising money for a defendant accused of a violent crime.

After months of silence, Mr. Hapgood recently spoke out about the case, repeating his claim of self-defense both outside the Anguilla courthouse and at a news conference in Manhattan.

“We’re hanging on by a thread, to be honest with you,” he told reporters. “It was a terrifying incident.”

The revised autopsy suggests that Mr. Mitchel may likely have died regardless of where he was that afternoon.

Different people react to cocaine differently depending on a variety of factors. But it is a widely accepted finding that 900 nanograms of cocaine per milliliter in the bloodstream is potentially lethal.