The New York City medical examiner's office says it will not release the cause or manner of death of Russia's ambassador to the United Nations.
Vitaly Churkin died suddenly last month at work, a day before he turned 65.
An autopsy was performed on the ambassador last month, but the sudden death required further study.
The additional tests have been completed, but Julie Bolcer, a spokeswoman for the city'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 autopsy 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, the Department of State noted the Russian Federation raised concerns after the autopsy 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 cause of Mr Churkin's 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.
James 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, 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 on his coffin.