By COLLEEN LONG and JONATHAN LEMIRE, Associated Press

NEW YORK (AP) — 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 city official briefed by the medical examiner's office.

The official was not authorized to reveal the cause of death for Ambassador Vitaly Churkin and spoke to The Associated Press on the condition of anonymity Friday after the medical examiner's office, citing diplomatic protocol, said it was instructed not to publicly release the cause of death.

An autopsy was performed on Churkin last month, but the death required further study. The additional tests had 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 U.S. Department of State asked the city in writing on Feb. 24 not to reveal the autopsy results because 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 counselor for host country affairs for the U.S. mission to the United Nations.

In a follow-up letter on March 1, 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," Donovan wrote in the follow-up letter to Democratic Mayor Bill de Blasio's international affairs office.

A spokesman for Russia's U.N. Mission declined to comment on the emergence of 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.

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."

Churkin, who died Feb. 20 at a hospital at age 64, had been Russia's envoy at the U.N. since 2006. He was the longest-serving ambassador on the Security Council, the U.N.'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 Churkin by President Vladimir Putin, was displayed at his coffin.

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Associated Press writer Edith M. Lederer at the United Nations contributed to this report.