Dutch prosecutors were Thursday investigating the security lapses that allowed a Bosnian Croat war criminal to commit suicide in front of shocked UN judges, in scenes that have cast a shadow over the court's two-decade legacy.

Staff at the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague had been expecting to spend the next few weeks quietly winding down the court which closes on December 31, after the judges delivered their final ruling on Wednesday.

But courtroom one is now an active crime scene after former Bosnian Croat military leader Slobodan Praljak, 72, tipped a small brown bottle into his mouth and died shortly afterwards in hospital.

"Initial tests have shown that the substance that was in the glass was a chemical substance which can cause death," prosecutor Marilyn Fikenscher, from the prosecution service in The Hague, told AFP.

"An autopsy will be carried out shortly, as well as a toxicology examination of the body," Fikenscher said, adding she could not estimate how long the investigation would take as it was "something quite unprecedented."

Dutch prosecutors said in a statement late Wednesday "for the time being the inquiry will focus on assisted suicide and violation of the Medicines Act."

- Last act of defiance -

Praljak's last act of defiance was broadcast live around the world and came moments after judges rejected his appeal, upholding his 20-year jail term for atrocities committed in a breakaway Bosnian Croat statelet during the 1990s wars.

The shocking images drew the curtain on two decades of work at the court, set up in 1993 to try those responsible for the worst atrocities in Europe since World War II.

But it remains a mystery exactly what the former theatre and movie director, known for his forcible courtroom presence and outbursts, drank and how he managed to evade tight security to smuggle it into the tribunal.

There are also questions about how he acquired the substance and if it was in the fortress-like UN detention centre in Scheveningen where he was being held, or inside the tribunal building a few miles away.

Everyone entering the detention centre is subjected to security checks "irrespective of his or her status, nationality, function or age," according to the rules.

Everyone must pass through scanners, and there may also be "a search of clothing." Every item brought to the centre or sent by mail is inspected, or opened or X-rayed.

Defendants are allowed access to their medications administered under the supervision of the chief medical officer. And they are also allowed approved visits from personal doctors.

But according to the Dutch NRC daily, "many of the suspects are elderly, like Praljak, and suffering from all sorts of illnesses" and it was "not unusual that he could have brought his medication to hearings."

On the last day of ICTY proceedings, the judges handed down their final verdict, upholding jail terms ranging from 25 to 10 years in the appeal case of six Bosnian Croat political and military leaders.

- 'No state responsibility' -

Praljak's death caused shockwaves in Croatia, where he had been considered a hero by some.

But Croatian President Kolinda Grabar-Kitarovic said citizens had to admit crimes had been committed by fellow Croats in Bosnia.

"We Croats need to have the strength to admit that some of our fellow compatriots in Bosnia committed crimes and they have to be held responsible for them," she added.

A minute's silence was also held in the Croatian parliament for all those who died in the conflicts.

Croatian Prime Minister Andrej Plenkovic repeated his conviction that the verdict -- which found all six men to be key participants in a scheme to ethnically cleanse Bosnian Muslims from the area -- was "unjust".

But he said the UN court had tried "individuals and not states, and it does not speak about responsibility of a state".

"These men knew how manipulate power and to ensure everyone is watching," said Frederiek de Vlaming, an expert in international law at the University of Amsterdam.

"It's a kind of protest which we haven't seen before," she told the De Volkskrant newspaper.