The International Criminal Tribunal for the Former Yugoslavia in the Hague itself became a crime scene on Wednesday when a convicted war criminal committed suicide with poison he had smuggled into the courtroom.

Slobodan Praljak, a commander of Croat forces during the 1990s war in Bosnia, swigged poison from a flask moments after a panel of appeals judges upheld his 20 year sentence for war crimes on Wednesday morning.

He was pronounced dead two hours later. Dutch police declared the courtroom a crime scene and have opened an investigation.

Praljak, 72, was jailed in 2013 for his role in war crimes including a massacre of civilians in central Bosnia and the deliberate destruction of Mostar bridge during the siege of the city by Croatian forces in 1993.

He committed suicide when Carmel Agius, the presiding judge, read out a verdict rejecting his appeal.

Refusing to sit, he said: “Judges, Slobodan Praljak is not a war criminal. With disdain, I reject this verdict.”

Ignoring a request to sit down, he swigged from what appeared to be a small flask and announced: “What I drank was poison.”