Chief prosecutor Carla Del Ponte said she cannot rule out the suicide theory until she has seen a more detailed report.
Earlier, the UN war crimes court said Milosevic died of a heart attack, but refused to completely rule out the possibility he may have been poisoned or taken his own life.
There have been reports he had suspicious substances in his blood, and Serbian newspapers have claimed he was murdered.
Serbian President Boris Tadic has ruled out a state funeral for Milosevic, and said the former leader’s wife Mirjana Markovic, who faces abuse of power charges and lives in exile in Moscow, would not receive a pardon.
His office said a state funeral would be “completely inappropriate because of the role he played in Serbia’s recent history” as well as contrary to the wishes of the Serbian people.
Milosevic’s legal advisor Zdenko Tomanovic said on Sunday that his former client had addressed a letter to the Russian foreign ministry saying he feared he was being poisoned after receiving a medical report indicating large amounts of a drug used to treat tuberculosis or leprosy in his blood.
“Milosevic pointed out that during the last five years he had never used any such antibiotics, especially since he had never had leprosy or tuberculosis or any kind of infectious disease except for the flu,” Mr Tomanovic said.
After a day of swirling rumours over the death of the man at the centre of the bloody Balkans conflict, the tribunal in The Hague announced that initial autopsy results showed he had died of a myocardial infarction.
Commonly known as a heart attack, this is caused by a blood clot blocking a coronary artery, leading to the death of heart muscle.
A statement said pathologists had identified two heart conditions that he suffered from and which “would explain the myocardial infarction.”
Final test results due later
Milosevic was found dead in his cell on Saturday at the war crimes court, where he had been on trial since February 2002 on more than 60 counts of genocide, war crimes and crimes against humanity during the Balkans conflicts.
The charges included the Srebrenica massacre of 8,000 Muslim men and boys, the biggest single atrocity in Europe since World War II.
The court said a toxicological examination was still being carried out and a final autopsy report will be issued as soon as possible.
“It is too early to reach any sort of conclusion,” a tribunal spokeswoman, Alexandra Milenov, said when asked whether Milosevic had been poisoned.
“The investigations are still ongoing,” she said. “We don’t expect a final report for a couple of days.”
A tribunal statement said the former Serb leader’s body would be released to his family on Monday.
Markovic told a Serbian newspaper that the tribunal had effectively killed her husband “because they didn’t have enough (proof) to convict him, and they couldn’t let him go.”
“He was terribly exhausted and everything was pointing toward what happened on Saturday.”
“Perhaps there were other measures which shortened his life,” she went on, adding that she had last spoken to him late on Friday.
“He told me, ‘sleep well my darling. I’ll call you as soon as I wake up in the morning’.”
In Belgrade, Foreign Minister Vuk Draskovic said he is “ashamed” by how much praise Milosevic loyalists have been lavishing on their late idol.
Dozens of hardcore supporters queued to pay tribute in front of his party seat in central Belgrade, where a large photograph of Milosevic and a book of condolences lay on a table in the entrance hall.
“By promoting a serial killer into a national hero, his victims are murdered again and Serbia disgraces itself… as the state in which crime is a supreme virtue,” Mr Draskovic told Tanjug news agency.
UN chief war crimes prosecutor Carla Del Ponte said it is now more urgent than ever to arrest other war crimes fugitives: former Bosnian Serb army chief Ratko Mladic and political leader Radovan Karadzic.