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The dramatic moment that took place this week at The Hague seemed like something out of a Cold War spy novel: A war criminal's 20-year prison sentence was upheld, prompting him to declare his innocence, toss back a vial of liquid, proclaim he'd poisoned himself, and die shortly after.

Now, it appears that what Bosnian Croat military commander Slobodan Praljak said was poison may have been potassium cyanide.

Preliminary autopsy results released by Dutch officials Friday report that he died from heart failure and that potassium cyanide was found in his blood.

"This has resulted in a failure of the heart, which is indicated as the suspected cause of death," Dutch prosecutors said Friday, citing the results.

Only seconds after a U.N. judge confirmed his 20-year war crimes sentence on Wednesday, the 72-year-old former philosophy professor and theater director who later became a wartime general shouted: “Slobodan Praljak is not a war criminal. I am rejecting the court hearing.”

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He then titled his head back and consumed what may have been potassium cyanide from a small brown-glass bottle. He then announced, "I have taken poison."

The episode was streamed live on the court’s website and around the Balkans before a judge halted the hearing and Praljack was rushed to a nearby hospital in the Netherlands, where he later died.

In their ruling, the judges reaffirmed that Praljack was guilty of crimes including murder, persecution and inhumane treatment as part of a plot to establish a Croat entity in Bosnia in the early 1990s. They also reaffirmed the 20-year sentence initially handed to him in May 2013.

Related: War criminal dies after drinking poison as 20-year sentence upheld in court

Dutch authorities are investigating how Praljack smuggled the poison into the court.

Lit candles and flags are placed in tribute to General Slobodan Praljak in Zagreb on Nov. 30, 2017, after the Bosnian Croat war criminal took his own life in front of United Nations war crimes judges. AFP - Getty Images

Potassium cyanide disrupts the body’s ability to use oxygen, causing unconsciousness or death by suffocation, according to the Centers for Disease Control and Prevention. In addition to whole-body toxicity, ingesting potassium cyanide typically causes nausea and vomiting, abdominal pain and corrosion of stomach and esophagus lining, according to the CDC.

In a similar war crime-related suicide, the Nazi leader Hermann Goring, who was designated as Hitler’s successor and deputy in all of his offices, was able to obtain a cyanide capsule that he used to avoid execution during the 1946 Nuremberg trials.

Potassium cyanide was used in the 1982 Tylenol poisonings that killed seven people in Chicago and four suburbs in the space of three days.

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The incidents prompted a national scare in which an untold number of people threw their medicine away and stores nationwide removed Tylenol from their shelves. The poisonings later led to the widespread adoption of tamper-proof packaging, which has since become standard.

No suspect was ever been charged or convicted for the poisonings.