Goran Stoparic effectively put a bounty on his head by giving evidence against police officer Sasa Cvjetan in a day of sensational testimony in the Serbian capital, Belgrade.
"I've put a target on my back. Now I'm a big Serbian traitor and it's normal that I'm frightened," he said after informing the court that he had received death threats.
The allegation offered both an indictment of Serbia's indigenous war crimes trials and perhaps its best hope.
Stoparic and Cvjetan were part of a special police outfit known as The Scorpions which was active in Kosovo during the brutal 1998-99 war between Serbian forces and ethnic Albanian separatist guerrillas.
Cvjetan is accused of involvement in the massacre of 19 ethnic Albanian civilians, including women and children, in the eastern Kosovo town of Podujevo in March 1999. The war ended a few months later after NATO intervention.
Stoparic had earlier denied any knowledge of the massacre, but he changed his story on Wednesday after the trial was moved to Belgrade from a small southern town due to security fears.
He said that at the time of the incident he was standing outside the house where the massacre allegedly took place.
He heard bursts of gunfire from inside and then he saw Cvjetan and other Scorpions emerge from the yard.
"Just a minute later, which is how long the shooting took, they came out of the yard of the house of Halim Gashi where the massacre was committed, changing the clips in their automatic weapons," he told the court.
He said that on Monday he had refused to testify because Slobodan Medic, the commander of the Scorpions who he had also named as being present inside the house at the time of the shooting, had threatened him.
"I've put a target on my back. Now I'm a big Serbian traitor and it's normal that I'm frightened"
former Serb police officer
"Medic told me in the hallway of this court that there would be drastic consequences for giving one kind of evidence and advantages to giving another kind of evidence, so that was why I didn’t testify on Monday," he said.
Threat to truth
What changed his mind is unclear, but Stoparic said he was ready to pay any price to tell the truth at last.
"I have already been threatened but even if they kill me, I don't regret [telling the truth] because kids were killed there," he said.
Dragoljub Todorovic, a lawyer for the victims' families, told B92 radio that Stoparic's testimony was "strong evidence that could lead to new indictments".
Stoparic's brave testimony comes at a time of mounting resistance to the whole idea of the United Nations war crimes court at The Hague.
War criminal campaign
Former Yugoslav president Slobodan Milosevic and other Serbian leaders are on trial for their part in the Balkan wars in Bosnia, Croatia and Kosovo in the 1990s.
But the reformist Serbian government has shown increasing impatience with The Hague, especially chief UN prosecutor Carla Del Ponte, accusing her of fuelling nationalist sympathies in advance of a general election later this month.
Milosevic is one of four war crimes indictees who are running for parliament in the election, and some opinion polls show the ultra-nationalist Serbian Radical Party is the frontrunner in the polls.