"This resulted in a failure of the heart, which is indicated as the suspected cause of death," the prosecutors' statement said.
A toxicological test found that Praljak "had a concentration of potassium cyanide in his blood."
Potassium cyanide is a highly toxic substance, and the lethal dose is 200-300 mg.
On Wednesday, Praljak, 72, drank from a small bottle what he said was poison. The incident took place as a judge announced his 20-year jail sentence during an appeal hearing in the International Criminal Tribunal for the former Yugoslavia (ICTY).
"I just drank poison. I am not a war criminal. I oppose this conviction," Praljak said at a hearing that was broadcasted online.
Hours later, Praljak died in a hospital in The Hague.
On Friday, the tribunal said it had launched its own investigation of his death. Prosecutors will focus on how Praljak managed to obtain poison in the highly secured UN building.