PARIS, March 6 — Milan Babic, a wartime Serbian leader who was closely cooperating with prosecutors, committed suicide in a prison cell of the war crimes tribunal in The Hague, the court said Monday.
Mr. Babic, 50, was found dead in his cell on Sunday evening, the court said in a statement. A senior court official said he had hanged himself, but the court spokeswoman would not confirm that. The official requested anonymity because he was not authorized to speak on the subject.
Mr. Babic's death deprives the prosecution of one of its valuable insider witnesses, a onetime close ally of the former Serbian president, Slobodan Milosevic. He was one of the leaders of an ethnic Serbs' rebellion in Croatia in 1991, and was in close contact with the top of the Serb government in Belgrade.
But he later said he had been deceived by Belgrade and turned himself in and pleaded guilty to crimes against humanity. Since then, he has testified against three senior fellow Serbian politicians, including Mr. Milosevic, and he was to give evidence in three more trials.
"He was a star witness for the prosecution, a key insider, no question," said Edgar Chen, a lawyer who monitors trials at the tribunal for the Coalition for International Justice.
One crucial question, Mr. Chen said, was whether Mr. Babic's past testimony would remain part of the record. Court officials said it was up to the judges in each trial to decide.
Among Serbs, Mr. Babic was widely seen as a turncoat, and tribunal officials had been so concerned for his safety that they would not disclose where he was serving his 13-year sentence.
He was brought back to the Netherlands in February to testify against another Croatian Serb and former comrade, Milan Martic. His testimony was to finish Monday.
It was the second suicide at the prison. Another Croatian Serb, Slavko Dokmanovic, hanged himself with his tie in 1998.
In recent weeks, Mr. Babic described how the rebellion of Croatian Serbs had been run from Serbia and led by Mr. Milosevic.
At various times, Mr. Babic and Mr. Milosevic shouted insults at each other across the courtroom. At one point, Mr. Babic told his former ally: "You dragged the Serbian people into war. You brought shame upon the Serbian people. You brought misfortune on the Croatian people, on the Muslim people."