Rudolf Vrba, who as a young man escaped from Auschwitz and provided the first eyewitness evidence not only of the magnitude of the tragedy unfolding at the death camp but also of the exact mechanics of Nazi mass extermination, died on March 27 at a hospital in Vancouver, British Columbia. He was 81.

His wife, Robin, said he died of cancer.

After the war Dr. Vrba went on to become a distinguished medical researcher in Israel, Britain, the United States and Canada, writing dozens of papers.

But his greatest importance is as an author of a much different paper — one with diagrams of gas chambers and crematories. With remarkable specificity gained from camp jobs that gave him unusual access to various corners of Auschwitz, including the gas chambers, Dr. Vrba told the unknown truth about it.