When the Soviet army liberated the Auschwitz death camp 70 years ago many of the prisoners had been killed or marched away by the retreating Nazis. But among those left were some twin children - the subject of disturbing experiments by Dr Josef Mengele.
Vera Kriegel and her twin sister Olga were just five years old when they were taken from their village in Czechoslovakia to Auschwitz.
Transported in cattle cars which were so tightly packed that the dead were still standing, she recalls the "sheer terror" of arriving at the camp and treading on "dead people like steps" as she left the train.
New arrivals at the camp were sorted into the weak, who would be gassed straight away, and the strong, who would be made to work. But Mengele and his assistants were there too, looking for twins.
Vera, her sister, and her mother were taken straight to SS Captain Josef Mengele. He was intrigued, she says, by what he described as her mother's "perfect Aryan features" and blue eyes, while Vera's and her sister's were brown.
Mengele selected them for experimentation.
Another woman who remembers her arrival at the camp is Jona Laks, who was taken as a teenager from the Lodz ghetto. She was not immediately recognised as a twin and was initially sent off in the direction of the gas chamber - when her sister told Mengele they were twins he had her brought to his laboratory.
Josef Mengele was an assistant to a well-known researcher who studied twins at the Institute for Heredity Biology and Racial Hygiene in Frankfurt - he started working at Auschwitz in May 1943.
There he had an unlimited supply of twins to study, and he wouldn't get in trouble if they died.
According to Prof Paul Weindling of Oxford Brookes University, author of Victims and Survivors of Nazi Human Experiments, hundreds of children were used in Mengele's experiments.
"I found a record of a prisoner doctor and bacteriologist who was forced to work for Mengele that there were 732 pairs of twins," he says, and suggests the doctor was interested in genetics. "I think Mengele might have been interested in the inheritance of the propensity to having twins."
He believes many of the twins survived Auschwitz, although he thinks Roma twins were almost certainly killed.
Some of the children, now elderly, have little memory of the experiments, others have memories that may not be 100% accurate.
Jona Laks says Mengele removed organs from people without anaesthetic, and if one twin died the other would be murdered. Vera Kriegel says that he killed people with an injection to the heart, and then dissected them.
She remembers being ushered into his laboratory. "I was looking at a whole wall of human eyes. A wall of blue eyes, brown eyes, green eyes. These eyes they were staring at me like a collection of butterflies and I fell down on the floor."
The first experiment she was subjected to involved being kept in a small wooden cage with her sister and being given painful injections in her back - she doesn't know why, but thinks it may have been an attempt to change the colour of her eyes.
In another experiment, she says, the pair of them and more than 100 other twins were given injections of bacteria that cause Noma disease - an infection of the mouth or genitals, which causes boils and often turns gangrenous.
Some twins became feverish, and some died, she says. She also remembers Mengele reacting angrily when twins went missing - once when this had happened she stared him out to prove he could not completely dominate her.
As well as twins, Mengele experimented on dwarves, giants and Romas.
Moti Alon, who arrived in Auschwitz aged nine in 1944, remembers being forced to watch a dwarf and a Roma woman being made to have sex.
He remembers having a number tattooed on his arm. The same happened to his brother, though the tattooist made a mistake. "Instead of writing 17 they wrote 10 so they erased it and did some dots," he says.
For Menachem Bodner who arrived at the camp with his brother as a three -year-old, this number became his identity.
When he left the camp in 1945, he had no idea who he was.
With the help of Israeli genealogist Ayana KimRon and a Facebook page set up to help, he has recently discovered that his real name is Elias Gottesman and that he and his brother, named Jeno, were born in a small town east of Munkacs, then part of Hungary, now in Ukraine (and known as Mukacheve).
KimRon also discovered that his father had died in a camp and that his mother, Roza, had returned to Hungary following a death march from Flossenburg concentration camp - only then to be murdered in her home town in 1946 during an anti-Semitic riot.
Now aged 74, he continues to search for the twin brother he last saw when the camp was liberated in 1945.
On 26 January 1945, Vera Kriegel remembers, the guards "were in a big panic. So they poured petrol over the barracks and tried to destroy all the evidence."
Grabbing a big pack of family photos, Vera, her mother and sister, fled the camp, only to be caught and beaten and thrown back into the barracks.
The following day, Soviet troops entered Auschwitz. The soldiers, she says, "brought these striped coats and told us to put them on and roll up our sleeves, so we could show our numbers.
"They filmed us, the children. They wanted to know what happened to us [and] Mengele's experiments. Everything was written down."
As for Mengele, he fled West and was arrested by the US Army. But he had no SS blood group tattooed on his arm so he was released by a unit that was unaware that his name was on a list of major war criminals.
He worked as a farmhand in Bavaria before escaping to Argentina in 1949.
Though the West German authorities issued a warrant for his arrest in 1959, Mengele remained in South America before his death from drowning following a stroke at a holiday resort in Brazil in 1979. He was buried in Sao Paulo under the name Wolfgang Gerhard.
The children coped with the appalling ordeal of Auschwitz and Mengele's experiments in different ways.
Moti Alon, his mother and twin, eventually made their way back home, arriving in Budapest on 5 May 1945. He now lives in Israel. "I have no traumas, not from this," he says.
Vera Kriegel emigrated to Israel with her mother after the war, where she lives today. Seventy years later, she still has nightmares.
Jona Laks became an activist, the head of a group of Mengele twins. She has been back to Auschwitz many times, and says what she experienced there has never left her mind.
Menachem, the boy with no name, eventually returned to his home town in Ukraine.
"I told the driver to stop and got out of the car, and something was familiar to me, very familiar.
"I remembered the road, I remembered two Gestapo approaching or arriving from my right side... and then they come to my home."
Above all, he recalled his parents, carefree before the war and the Holocaust.
"It was noon. my mother wore a green skirt with white flowers... I remember her from the back, not the front.
"This is what I remember."
Watch Newsnight's report on the twins of Auschwitz - or watch a longer report on BBC World News at 23:30 GMT on Friday 30 January.
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