Investigators at the UN’s war crimes court in The Hague are scrambling to seal security breaches amid fears other inmates will “do a Praljak”, copying the Bosnian Croat warlord who dramatically killed himself with cyanide last week.

Slobodan Praljak’s suicide, carried out in the dock and televised around the world, threatens to make a mockery of the court’s vaunted security procedures.

With Dutch police opening their own investigation, the tribunal has appointed Gambia’s chief justice Hassan Jallow to find out how Praljak, 72, obtained the bottle of potassium cyanide that Netherlands prosecutors said killed him.

The investigation is likely to start with the most glaring breach – the lack of any guard placed close enough to Praljak to snatch the cyanide from his grasp.

Most Hague trials have a blue-uniformed guard in the dock beside defendants: two were deployed to flank the famously irascible Bosnian Serb general Ratko Mladić last month when judges convicted him of genocide for orchestrating the Srebrenica massacre.

Yet no guard was beside Praljak, who was sentenced to 20 years for a litany of crimes against Muslims during the Bosnian war, when he appeared in the dock along with four fellow Bosnian Croat defendants to hear appeals against hefty sentences rejected.

Just two security staff bookended the line of defendants in what was billed as the final act of the court before it closes its doors at the end of this month, and neither noticed the bottle Praljak held in front of him in his meaty hands.

After hearing his 20-year sentence confirmed by appeal judges, the bearded warlord declared “Slobodan Praljak is not a war criminal, I reject your judgment with contempt,” and thrust the contents of the bottle into his mouth.

Only when Praljak, tottering, his eyes glazed, slumped to his seat and announced “I have taken poison” did the judges react.

“There were normally two guards behind the accused … not this time,” said a Croatian official, who would not be named because they are not authorised to talk to the media. “The Croatian press says that perhaps the last day of the Tribunal contributed to the relaxing of rules.”

Investigators will want to know also how Praljak got the poison into the dock despite rules mandating strip searches and X-rays for defendants entering and leaving the court complex. Given the lethality of even small doses of cyanide, officials may count themselves lucky he used it only on himself.

Of concern to investigators is whether other convicts still awaiting appeals verdicts, by a successor court due to start work next month, may be tempted to take similar drastic action.

Suicide has long stalked the Hague tribunal, a byproduct of hefty sentences handed down for horrific crimes in the wars of Bosnia, Croatia and Kosovo.

Two Hague inmates, both Serbs, have hanged themselves in the cells of the court’s Scheveningen prison on the Dutch coast. Slobodan Milošević, Serbia’s former president and the Hague’s most high-profile defendant, was apparently able to smuggle an antibiotic into his jail cell to exacerbate a heart condition that killed him near the end of his three-year trial in 2006.

Visitors to the Hague’s fortress-like court building are subjected to X-ray searches, and liquids are supposedly banned, but judges have not introduced more intrusive body-cavity searches, fearing a public backlash.

“It’s like the airport security, it’s not all that intense,” says Dr Eric Gordy, of University College of London’s school of Slavonic and eastern European studies. “If the searches were more intense everybody would complain.”

Some fear the suicide will tarnish the successes of a court that has jailed dozens of war criminals and achieved a string of precedents in international justice, convicting those responsible for atrocities including ethnic cleansing, mass rape and the siege of Sarajevo.

Praljak, a theatre director who became a key leader of Bosnian Croat forces in the early 1990s, was convicted for crimes including the bombardment of the ancient town of Mostar and prisoner abuses. Yet the verdict has been declared a “deep moral injustice” by Croatian prime minister Andrej Plenković, believed to be the first declaration by the head of an EU government in support of a convicted war criminal. “This [suicide] will just cement what was already public opinion about the tribunal,” says Mina Vidaković of Sense news agency which is archiving Hague trial material. “In Serbia and Croatia the establishment and the media are more concerned about the accused than the victims.”

Others think the court’s legacy will survive. “I don’t think this strange and bizarre event will distract from the bigger story of the accomplishments of the tribunal,” said Balkans expert Tim Judah. “Its successes outweigh its failures.”