Eva Kor was just a normal little girl growing up in Romania in 1944. She had three sisters including a twin, Miriam. Things were fine through the first part of World War II. Then, when Nazis invaded her hometown, everything changed. Nazi teachers took over the one-room schoolhouse for peasant children, and pushed propaganda in every lesson, incorporating things like killing Jews into every lesson, Eva said in an interview with the Indy Star.

When that wasn’t enough, they forced people onto cattle cars and took them to the Auschwitz concentration camp. As they stepped off the train, people were sorted into two groups—the ones who would live and the ones who would die.

In the confusion, Eva and Mirian were separated from the rest of their family. The twins had something that one of the Nazi doctors wanted—identical genetics. That doctor—and we use that term loosely—was Josef Mengele, known also as the Angel of Death.

Mengele was obsessed with twins, ultimately pulling 3,000 of them from the one million people who would enter Auschwitz. Only 200 would survive the war. He would perform experiments on them since he had an identical control group to measure results against.

He took them to a special part of the concentration camp where the first thing they saw was the bodies of three children on the bathroom floor. Eva decided right then that she would not end up dead on a dirty lavatory floor like those children.

Eva was used in two types of experiments at the hands of Mengele. Three days a week, the Nazis would take the clothes off of Eva and Miriam and put them in a room with other twins. They measured every part of the twins’ bodies and compared it to their siblings. Those measurements were then compared to the charts of other children, Eva said in an interview with Buzzfeed.

On the other days of the week, Eva and Miriam were taken to the blood lab and injected with mystery serums. To this day, Eva doesn’t know what she was injected with. After one injection, Eva became so ill that she was only expected to live for two more weeks. She pulled through, though, and returned to the barracks to live with her sister.

Whatever Mengele injected Miriam with came back to haunt her years after the Holocaust when they found out that she didn’t respond to antibiotics.

In January 1945 the camp was liberated, and Eva and Miriam went to Israel to live. Miriam died in 1993 as a wife and mother of three.

Here’s where the story changes. The horrors of the Holocaust are too much to understand, but what is really inexplicable is that Eva eventually forgave Mengele and the Nazi doctors.

A professor in Boston asked Eva to come speak to the students. Not only that, he asked her to bring a Nazi doctor—the same people who performed experiments on her and killed the rest of her family and—with her. Eva reached out to Hans Münch, one of the doctors she had seen on a documentary the year before. He wouldn’t go to Boston with her but he did agree to talk at his home in Germany.

When she met with him, she asked him to go with her back to Auschwitz and sign a document detailing what he did and saw in the camp. It’s a small victory for the survivors of the Holocaust—something to use in their arguments against Holocaust-deniers.

Eva wanted to show her appreciation for his willingness to attest to the crimes at the camp—particularly at the gas chambers.

“I didn’t know how to thank a Nazi,” Eva said. “I didn’t tell anyone about it, because even to me, it sounded strange…After 10 months, one morning I woke up and the following simple idea popped into my head: How about a letter of forgiveness from me to Dr. Münch.”

As she was writing that letter, Eva learned something new about herself. She had the power to forgive someone who was responsible for killing so many people.

“No one could give me that power,” Eva said. “No one could take it away. It was mine to use in any way I wished.”

The letter came 50 years after the liberation of Auschwitz. At first Eva didn’t know where to start—what do you even say in those circumstances? It took her four months to complete the letter to Münch.

Eva’s former English professor proofed her letter. It was this teacher who proposed an even bigger act of forgiveness.

“She said to me, ‘Now Eva, you forgive this Dr. Münch. Your problem is not with Dr. Münch. Your problem is with Dr. Mengele,’” Eva said.

Understandably, she wasn’t quite ready to forgive him. The professor gave her some homework—she wanted Eva to go home and pretend like she was talking to Mengele, specifically forgiving him for what he had done. Eva went home and used a dictionary to find 20 mean words, words she thought described Mengele and the horrors he created. Then, she read the list out loud to the imaginary Mengele in her room.

“At the end of that I said, ‘In spite of that, I forgive you,’” Eva said. “Made me feel very good that I, the little guinea pig of 50 years, even had the power over the Angel of Death of Auschwitz.”

Only after forgiving Mengele and the other Nazis who oversaw Auschwitz did Eva finally feel free. Other survivors were angry with Eva for forgiving them, but she knew that it was the right thing to do.

“I want everybody to remember that we cannot change what happened,” Eva said. “That is the tragic part. But we can change how we relate to it.”

You can watch the full interview with Buzzfeed here.