The cause and manner of death of Russia’s ambassador to the United Nations needs to be studied further, New York’s medical examiner said.
It comes a day after diplomat Vitaly Churkin fell ill at his office at Russia’s UN mission in the city and died in hospital.
Further study usually includes toxicology and other screenings, which can take weeks.
The case was referred to the medical examiner’s office by the hospital, spokeswoman Julie Bolcer said.
Mr Churkin, who died a day before his 65th birthday, 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.
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.
Most of the deaths investigated by the office are not suspicious.
The Security Council held a moment of silence on Tuesday in memory of Mr Churkin, whom UN secretary-general Antonio Guterres called "not only an outstanding diplomat but an extraordinary human being".
Russian President Vladimir Putin esteemed Mr Churkin’s "professionalism and diplomatic talents", spokesman Dmitry Peskov said, according to state news agency TASS.
Moscow has not yet given a date for the funeral.
Diplomatic colleagues from around the world mourned Mr Churkin as a master in their field, saying he had both a deep knowledge of diplomacy and a large and colourful personality.
US ambassador Nikki Haley said that while she and Mr Churkin did not always agree, "he unquestionably advocated his country’s positions with great skill".
Her predecessor, Samantha Power, described him on Twitter as a "diplomatic maestro and deeply caring man" who had done all he could to bridge differences between the US and Russia.
Those differences were evident when Ms Power and Mr Churkin spoke at the Security Council last month, and Ms Power lashed out at Russia for annexing Ukraine’s Crimean Peninsula and for carrying out "a merciless military assault" in Syria.
Devastated by passing of Russian UN Amb Vitaly Churkin.Diplomatic maestro &deeply caring man who did all he cld to bridge US-RUS differences— Samantha Power (@SamanthaJPower) February 20, 2017
Mr Churkin countered that Democratic former president Barack Obama’s administration, which Ms Power served in, was "desperately" searching for scapegoats for its failures in Iraq, Syria and Libya.
Mr Churkin died weeks into some major adjustments for Russia, the UN and the international community, with a new secretary-general at the world body and a new administration in Washington.
Meanwhile, the Security Council is due this week to discuss Ukraine and Syria.
From Moscow’s vantage point, "Churkin was like a rock against which were broken the attempts by our enemies to undermine what constitutes the glory of Russia", TASS quoted deputy foreign minister Sergei Ryabkov as saying.
Mr Churkin’s UN counterparts "experienced and respected the pride that he took in serving his country and the passion and, at times, very stern resolution that he brought to his job", said General Assembly president Peter Thomson, of Fiji.
But colleagues also respected Mr Churkin’s intellect, diplomatic skills, good humour and consideration for others, said Mr Thomson, who called for a moment of silence at the start of an unrelated meeting on Monday.
Mr Churkin emerged as the face of a new approach to foreign affairs by the Soviet Union in 1986, when he testified before the US congress about the Chernobyl nuclear plant disaster.
It was rare for any Soviet official to appear before congress.
In fluent English, Mr Churkin provided little new information about Chernobyl but engaged in a friendly, sometimes humorous, exchange with politicians who were not accustomed to such a tone - or to a representative in a fashionably well-fitting suit with a stylish haircut - from the Soviet Union.
After he returned to the foreign ministry in Moscow, he ably dodged questions and parried with Western correspondents, often with a smile, at briefings in the early 1990s.
Within the government, he proved himself an able and flexible presence who survived numerous course changes after the dissolution of the Soviet Union.
He held ambassadorships in Canada and Belgium, among other posts.
Mr Churkin told Russia Today in an interview this month that diplomacy had become "much more hectic", with political tensions rising and stability elusive in various hotspots.
At the time, he looked in good health, reporter Alexey Yaroshevsky tweeted.