Convicted Bosnian Croat war criminal Slobodan Praljak died after taking potassium cyanide in a war crimes court in The Hague, Dutch officials have said.

“The first preliminary result from the toxicology report shows that Mr. Praljak had a concentrated amount of potassium cyanide in his blood,” the Public Prosecutor’s Office said in a statement on Saturday.

“That led to heart failure, which is the probable cause of death,” the statement added.

A confirmation regarding the cause of Praljak’s death on Wednesday is expected in a final report after the end of the investigation.

The probe is also looking at how he managed to get the vial of poison into the high-security building.

Praljak drank what he said was poison just seconds after hearing he had lost his appeal against a 20-year-prison sentence.

“I, Slobodan Praljak, reject the verdict. I’m not a war criminal.”

Then he drank from a small bottle or flask and declared: “What I am drinking now is poison.”

The judge immediately suspended the hearing and called for a doctor.

The incident happened when the International Criminal Tribunal for the former Yugoslavia (ICTY) was handing down its last judgment in an appeal by six Bosnian Croat political and military leaders, who were convicted in 2013 of persecuting, expelling and murdering Bosnian Muslims during the 1992-1995 war.

Wednesday’s hearing was the final case to be completed at the ICTY before it closes its doors next month.

The tribunal, which last week convicted former Bosnian Serb military chief General Ratko Mladic of genocide and other crimes, was set up in 1993 while fighting still raged in the former Yugoslavia.

It indicted 161 suspects and convicted 90 of them.