Bosnian Croat war criminal Slobodan Praljak died after taking potassium cyanide in The Hague's courtroom, Dutch prosecutors say, citing the results of a preliminary post mortem examination.
"This has resulted in a failure of the heart, which is indicated as the suspected cause of death," they said.
Praljak drank from a vial after his 20-year term for war crimes in Bosnia in the 1990s had been upheld on Wednesday.
Dutch authorities are investigating how he smuggled poison into the court.
The confirmation rulings against Praljak and five other defendants brought an end to more than 20 years of work by the International Criminal Tribunal for the former Yugoslavia (ICTY).
What happened in the courtroom on Wednesday?
Seconds after hearing his appeal had been lost, the former general declared: "Slobodan Praljak is not a war criminal. I am rejecting the court ruling."
The 72-year-old then drank from a small brown-glass bottle and announced: "I have taken poison."
The presiding judge cut short the hearing and Praljak was taken to hospital.
Officials said Praljak died shortly afterwards in a local hospital.
How tight is security?
Peter Robinson, a lawyer for convicted Bosnian Serb war criminal Radovan Karadzic, told the BBC he could not understand how anyone could have passed poison to a prisoner.
"We go through two security checks when we visit detainees," he said. "Two metal detectors. You can't bring drinks, not even a bottle of water or Coke. Everything is searched before you go in.
"I've only ever seen him [Praljak] with his family, in the room next door to us. But when his family came it was private.
"There are no guards inside the room at the time, they stay outside the door. You can see your own doctors with special permission."
Who was Praljak?
Though allies against the Bosnian Serbs during the 1992-95 civil war, Bosnian Croats and Muslims also fought each other for a period of 11 months, with the city of Mostar seeing some of the fiercest fighting.
Praljak was convicted in 2013 of crimes against humanity, committed while commanding Bosnian Croat forces. The ICTY found that he had:
- Failed to make any serious efforts to stop his soldiers rounding up Muslims in the summer of 1993
- Failed to act on information that murders were being planned, as well as attacks on members of international organisations and the destruction of Mostar's historic Old Bridge and mosques
However, it did allow part of Praljak's appeal, finding that Mostar's Old Bridge had been a "military target at the time of the attack".
Why do some regard him as a hero?
Croatian Prime Minister Andrej Plenkovic said his death spoke of a "deep moral injustice towards six indicted Croats from Bosnia and the Croatian people".
However, Croatian President Kolinda Grabar-Kitarovic went on television to say Croats needed to admit that some of their compatriots had committed crimes in Bosnia.
In Bosnia itself, Dragan Covic, the Croat member of the tripartite presidency, said his suicide showed "what sacrifice he was ready to make" to show he was not a war criminal.
About 1,000 Bosnian Croats gathered in a Mostar square late on Wednesday to light candles in memory of Praljak.