Russia's ambassador to the United Nations, who collapsed in his office last month, died from a heart attack, and no foul play is suspected, according to a senior official briefed by the medical examiner's office.
The official was not authorised to reveal the cause of death for Ambassador Vitaly Churkin and spoke on condition of anonymity after the medical examiner's office, citing diplomatic protocol, said it was instructed not to publicly release the cause.
A post mortem was performed on Mr Churkin last month, but the death required further study.
The additional tests had been completed, but Julie Bolcer, a spokeswoman for New York's medical examiner, said the city's Law Department told the office not to release any further information "to comply with international law and protocol".
The US Department of State asked the city in writing on February 24 not to reveal the post mortem results because Mr Churkin's diplomatic immunity survives his death.
"The United States insists on the dignified handling of the remains of our diplomatic personnel who pass away abroad (including in Russia) and works to prevent unnecessary disclosures regarding the circumstances of their deaths," wrote James Donovan, minister counsellor for host country affairs for the US mission to the United Nations.
In a follow-up letter on March 1, the Department of State noted the Russian Federation had raised concerns after the post mortem had been conducted and "voluntary statements reported in the media about Ambassador Churkin's medical history" prompted complaints from Russian diplomats.
"The information reported was very private in nature and included information about which even they had no knowledge," Mr Donovan wrote in the follow-up letter to Democratic Mayor Bill de Blasio's international affairs office.
A spokesman for Russia's UN Mission declined to comment on the emergence of Mr Churkin's cause of death.
The spokesman earlier praised the city Law Department for asking that it not be released, saying the department's guidance "fully complies with the principles of inviolability of private life and diplomatic immunity".
The medical examiner is responsible for investigating deaths that occur by criminal violence, accident, suicide, suddenly or when the person seemed healthy or if someone died in any unusual or suspicious manner. City policy is to publicly release the cause of death.
Mr Donovan argued state policies could be overruled by federal authority where "it creates an obstacle to the achievement of the president's foreign policy as reflected in an international agreement".
Mr Churkin, who died on February 20 at a hospital aged 64, had been Russia's envoy at the UN since 2006. He was the longest-serving ambassador on the Security Council, the UN's most powerful body.
He was buried in Moscow, where he was praised by Foreign Minister Sergey Lavrov as an "exceptional professional and people's diplomat".
The Order of Courage, a medal awarded posthumously to Mr Churkin by President Vladimir Putin, was displayed by his coffin.