Milan Babic, the Serb leader of a rebel republic in Croatia and one of the key figures in the Balkan wars of the 1990s, committed suicide in prison, the U.N. war crimes tribunal said Monday.

Babic, who was serving 13 years for crimes against humanity, was found dead Sunday evening in his cell at the U.N. detention center in Scheveningen, a suburb of The Hague, said a tribunal statement.

The Serb minority revolted after Croatia broke away from Yugoslavia in 1991, setting off a war that lasted until 1995.

With the support of Slobodan Milosevic, the Yugoslav president, Croatian Serbs proclaimed their own republic, with Babic as its president, and began an ethnic cleansing campaign against non-Serbs.

Babic later was a pivotal witness at Milosevic's war crimes trial, which is continuing in The Hague.

Babic's family was informed Sunday after the chief medical officer of the center confirmed the cause of death was suicide. Dutch authorities were notified and a tribunal judge immediately ordered an inquiry.

The tribunal did not say how Babic killed himself. In Belgrade, the B92 television station said Babic "probably hanged himself."

It was the second time a detainee committed suicide. The first was Slavko Dokmanovic, another Croatian Serb leader, in 1998.

Babic, 50, pleaded guilty in 2004 to a single charge of inflaming an ethnic cleansing campaign that killed hundreds of Croats and expelled tens of thousands in the self-proclaimed Republic of Serbian Krajina.

The plea was part of a deal in which prosecutors dropped four other charges of murder, cruelty and the wanton destruction of villages during the war in Croatia.

His appeal against the lengthy sentence was rejected last July and he was transferred to another prison to serve his sentence.

He returned to The Hague last month to testify against Milan Martic, who became the leader in Krajina after Babic broke with Milosevic, his former mentor.

Babic was accused, along with Milosevic, of being part of a conspiracy, or "joint criminal enterprise," to clear roughly one-third of Croatia of non-Serbs and incorporate that area into an ethnically pure Serbian state.

Unlike Milosevic — who rejects the legitimacy of the U.N. court and was extradited to The Hague by Serbian authorities — Babic voluntarily surrendered to face the charges.

In his plea agreement, Babic said he knew nothing of the murders of non-Serbs until many years later and had no knowledge at the time of the extent of the crimes being committed by Serb forces.

Before sentencing, he apologized to the Croatian people.

"I stand before this tribunal with a deep sense of shame and remorse. I allowed myself to participate in the persecution of the worst kind against people only because they were Croats, not Serbs," he said. "I ask my brother Croats to forgive us, their brother Serbs."

Babic's three-week-long testimony in 2002 against Milosevic provided a dramatic highlight to a trial that has gone on for more than four years.

Though Milosevic denied involvement in the Serb uprising in Croatia, Babic told the court he in fact played a key political and military role behind the scenes and had "pulled the strings" of the Krajina Serbs.

The two men hurled accusations at each other while the three U.N. judges looked on.

"You dragged the Serb people into war," Babic said, charging that Milosevic failed to protect the Serbs. "You brought shame on the Serbs."

Babic fled with his family to Belgrade in 1995 on the eve of a Croatian army offensive which retook Krajina and other Serb areas, along with 250,000 other Croatian Serb refugees. He stayed in a Belgrade suburb where he grew mushrooms on a cousin's farm.