Tribunal officials in The Hague and the police in the witness’s new country of residence refused to discuss her case. But her story was largely corroborated by court documents and letters she showed, and by lawyers who dealt with the case, providing a rare look into an important but deeply hidden part of modern international justice.

ALL modern war crimes tribunals depend heavily on witnesses because large-scale killings, torture or rape rarely come with a paper trail leading to those responsible.

Many testify under pseudonyms, but only a small portion of witnesses are deemed so endangered that they are secretly moved to another country and given new names.

Their numbers and whereabouts are closely held secrets, and next to nothing is publicly known about how they fare. They are told they will be cut loose from the program if they reveal their original identities. Journalists are warned that they risk prosecution for contempt of court if they disclose the name or the location of any protected witness, and some have been convicted.

“Witness protection is an incredibly important issue,” said Herman von Hebel, the chief administrator of the International Criminal Court, while attending a recent panel discussion in The Hague on witness safety. “Without witnesses, without key insiders, there are basically no trials.”

Witness B-129 was such an insider. For two days, she transfixed the court during the Milosevic trial as she described her work as a trusted aide to Zeljko Raznatovic — better known as Arkan — a shady Serbian entrepreneur with arrest warrants across Europe.

In the Balkan wars of the early 1990s, Mr. Raznatovic created a militia, the Tigers, or Serbian Volunteer Guard. They quickly became notorious for smuggling, looting and killing in neighboring Croatia and Bosnia. Mr. Milosevic claimed during his trial in The Hague that he knew nothing about the Tigers’ activities.