Former Croatian Serb leader Milan Babic has committed suicide in his prison cell in The Hague, the UN war crimes tribunal has said.
Babic, 50, was serving a 13-year prison term for crimes against humanity, after admitting persecuting the non-Serb population in Croatia's Krajina region.
He was a key ally of former Yugoslav President Slobodan Milosevic but later testified against him at the tribunal.
He was found dead on Sunday evening, the tribunal said in a statement.
BABIC: RISE AND FALL
1990 : Mayor of Knin
May 1991 - Feb 1992 : President of Krajina (first an "autonomous region" then a "republic") later foreign minister and prime minister
1995 : Flees with other Serbs as Croatian army takes Krajina
2002 : Testifies against Slobodan Milosevic
2003 : Surrenders to tribunal
2004 : Pleads guilty to persecuting non-Serbs, sentenced to 13 years
2005 : Loses appeal against sentence
2006 : Commits suicide
"The Dutch authorities were called immediately. After conducting an investigation, they confirmed that the cause of death was suicide," the statement said.
It did not say how Babic had killed himself.
A spokeswoman for the tribunal, Alexandra Milenov, told the BBC that, like all detainees, Mr Babic was subject to regular monitoring.
"He was checked at six o'clock and everything was found to be all right and at six-thirty when he was checked again his body was found."
A tribunal judge has opened an inquiry.
Babic feared retribution for testifying against Slobodan Milosevic in 2002, and had been had been serving his sentence at a secret location, at his own request.
There was nothing unusual in his demeanor
Tribunal spokeswoman, Alexandra Milenov
He was brought back to The Hague last month to testify in the trial of Milan Martic, his successor as Croatian Serb leader.
Babic was president of the self-declared breakaway Krajina Serb republic, covering about one-third of Croatian territory, after Croatia declared independence from Yugoslavia in 1991.
He was jailed in 2004 for crimes committed during Croatia's 1991-1995 war.
In return for his guilty plea, prosecutors dropped four other charges of murder, cruelty and the wanton destruction of villages.
Last year, judges rejected an appeal against his lengthy sentence.
A dentist by profession, Mr Babic had expressed shame and remorse over his actions in Krajina.
He reportedly reaffirmed this in February saying his guilt was a "pain that I have to live with for the rest of my life".
However, tribunal spokeswoman Alexandra Milenov said he had given no indication that he was contemplating suicide.
"There was nothing unusual in his demeanor," she said.
It is the second time a detainee in The Hague has committed suicide. The first was Slavko Dokmanovic, another Croatian Serb leader, in 1998.