The Russian ambassador to the UN who collapsed in his office in New York last month reportedly died from a heart attack.
No foul play was suspected in the death of Vitaly Churkin, according to an unnamed senior official briefed by the medical examiner's office who spoke to the AP news agency.
While a post-mortem examination was completed last month, further tests apparently needed to be carried out.
But a spokeswoman for New York's medical examiner, Julie Bolcer, said the office had been told by the city's Law Department not to give any more details "to comply with international law and protocol".
America's Department of State has requested in writing that the post-mortem results are not revealed because Mr Churkin's diplomatic immunity survives his death.
In another letter, the department said that "voluntary statements reported in the media about Ambassador Churkin's medical history" had led to complaints from Russian diplomats.
James Donovan, minister counsellor for host country affairs for the US mission to the UN, told New York Mayor Bill de Blasio's international affairs office: "The information reported was very private in nature and included information about which even they had no knowledge."
Regarding Mr Churkin's reported cause of death, a spokesman for Russia's UN Mission declined to comment.
Mr Churkin, who was 64 when he died on 20 February , had been Russia's envoy at the UN since 2006.
Russian foreign minister Sergey Lavrov said he had been an "exceptional professional and people's diplomat".