Medical examiners who performed an autopsy on Russia's ambassador to the United Nations said Tuesday that more tests are needed to determine how and why he fell ill in his office and later died at a hospital.

Vitaly Churkin, who died Monday 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.

The city's medical examiners concluded Churkin's death needed further study, which usually includes toxicology and other screenings. Those can take weeks.

The medical examiner is responsible for investigating deaths that occur by criminal violence, by accident, by suicide, suddenly or when the person seemed healthy or in any unusual or suspicious manner. Most of the deaths investigated by the office are not suspicious.

Churkin's case was referred to the medical examiner's office by the hospital.

Moscow has not given a date for Churkin's funeral.

Diplomatic colleagues from around the world mourned Churkin as a master in their field, saying he was deeply knowledgeable about diplomacy and dedicated to his country while also being a personable and witty colleague.

"He could spot even the narrowest opportunities to find a compromise," U.S. Ambassador Nikki Haley said Tuesday, calling Churkin "brilliant, wise, gracious and funny."

Ukraine holds the Security Council's rotating presidency, and Ukrainian Ambassador Volodymyr Yelchenko led members Tuesday in a moment of silence in Churkin's memory. Yelchenko didn't add his own words to the tributes that followed, though Ukrainian Foreign Minister Pavlo Klimkin gave condolences when reporters asked afterward.

While noting "fundamental differences" with Churkin's political positions and how he presented them, that divide "does not put us away from our point of expressing human condolences," Klimkin said.