They left this camp of horrors 40 years ago as children, taking with them a lifetime of nightmares, and returned today from the United States and Israel, a special group of Auschwitz survivors who found one another and the courage to confront the past.

They are the twins selected by Nazi doctor Josef Mengele to be human guinea pigs in experiments that also included dwarfs and cripples and were meant to find a faster way of producing a superior, defect-free race.

With a handful of other former inmates, they gathered under gray skies on the snow-covered platform of the rails that brought them to the world's largest death camp. Telling of their arrivals, one by one they recalled stepping from cattle cars into a brightly lit scene of SS guards, barking dogs, shouting officers and military order.

Mengele, tall, slim and commanding, stood waiting and watching on the platform. His men plucked the twins immediately from the masses of new arrivals.

In all, 1,500 pairs underwent tests that to this day remain largely a mystery to the victims. Less than 180 individual twins survived.

Only one set came back today -- Eva Moses Kor of Terre Haute, Ind., and her fraternal twin, Miriam Moses Czigher of Israel. The other six twins came alone, either because their siblings had died, did not want to come or could not afford the journey.

More are expected to appear before a public tribunal in Jerusalem starting Feb. 4 to document Mengele's experiments and spur the search for the doctor, who is believed to be alive. Simon Wiesenthal and other veteran Nazi hunters say Mengele, who would be 73 now, lives in Paraguay under the protection of longtime President Alfredo Stroessner, but the Paraguayan government repeatedly has denied this.

All present today said they had decided to return to Auschwitz on this, the 40th anniversary of the camp's liberation by Soviet troops, to make sure the world does not forget the 4 million who historians say died here or the need to bring Mengele to justice.

Wearing bright yellow stars in remembrance of the badges the Nazis forced on Jews in the camp and in ghettos under German occupation, the survivors placed blue-and-white paper wreaths in the snow on the railway platform, lit six blue candles and sang "Hatikva," the Jewish song of hope, then moved into the camp, which has been preserved much as the Soviets found it.

In place of the tattered striped uniforms and wooden clogs they wore four decades ago, the former inmates were dressed warmly in down and woolen coats, fur hats and insulated boots against a biting Polish cold.

Wandering across the bleak and eerie expanse of Birkenau, the sprawling extension two miles from the Auschwitz mother camp that served as the site of arrivals and most of the gassing and cremating, they felt memories rush to the surface.

Marc Berkowitz, 52, of New City, N.Y., dashed up to the high barbed-wire fence that surrounded the camp and was once electrified. "Look! No more electricity," he exclaimed, wrapping his hand around a bit of wire.

Vera Kriegel, only 5 when she arrived in 1943, was overcome with the memory of her father, whom she last saw being led away at the train ramp.

"Rest in peace," she said tearfully, as if speaking to him. "Maybe you see us here wherever you are. We are remembering you."

Kor and Czeigher led a trail of journalists into the bare musty bathhouse where new inmates were tattooed and given camp garb. Kor, 50, said she had resisted the tattoo, so four guards had to hold her in place.

Polish authorities declared Auschwitz -- now known once again by the Polish name Oswiecim -- a national monument in 1947. The German slogan "Arbeit Macht Frei" (Work Sets You Free) fills the arch over the entrance to the main camp. Old, red-brick barracks house museum exhibits chronicling the mass execution of prisoners, the majority of them Jews, from more than two dozen countries.

In Birkenau, the wooden barracks that housed male inmates has crumbled away. Only rows of brick smokestacks, which hardly provided warmth four decades ago, remain, like a forest of bare poles. At the far end of the camp are the ruins of the gas chambers and crematoriums, great heaps of caved-in burnt bricks and twisted metal rods left from an attempt by the Nazis to blow up the death factories and cover the evidence.

Facing the charred remains of crematorium No. 2, the twins and other survivors, some of them weeping, recited kaddish, a Jewish prayer for the dead. Then, holding flowers, they stood on the steps of an international monument dedicated to the dead. Dov Shilansky, a Likud member of the Israeli parliament, challenged those who claim that there was no Holocaust here to come view the evidence.

Holocaust experts say they have noticed a rise in the number of such claims, promoted by extreme right-wing groups in West Germany and the United States.

Locking arms under a gray banner saying "I Accuse," the survivors ended the day retracing the "death march" from Birkenau to Auschwitz made by tens of thousands of evacuated inmates in the final hours before the Soviets reached the camp.

No Polish government representative was present today. Local dignitaries held an anniversary ceremony yesterday attended by about 120 Polish survivors of Auschwitz. Polish authorities are waiting until the end of March to stage a major commemoration of all the death camps built by the Nazis in Poland.

Records of the experiments done by Mengele at Auschwitz are scarce. The twins remember that he often drew blood from them, sometimes attempting transfusions between twins. He performed frequent measurements, experimented on genitals, tried to change eye coloring, injected various chemicals and, when finished, killed patients with shots of phenol in the heart to preserve them for dissection.

"Mengele injected all sorts of things in us," said Vera Kriegel. "We don't know to this day what we got."

Many of the twins say they still suffer medical problems, ranging from chronic urinary tract infections to spinal difficulties, which they believe are side effects from the experiments. Many, too, frequently experience nightmarish flashbacks -- seeing a smokestack, for instance, and thinking of a crematorium or associating certain smells with the stench of dead bodies.

For years, the twins had no idea how many of those who had been treated by Mengele had survived. But two years ago, at a Holocaust commemoration in Washington, Kor and Berkowitz met for the first time and decided to establish an organization in the United States and in Israel to help unite those Auschwitz twins still alive.

Explaining why they had decided to return now to the scene of their childhood terror, several of the twins cited a desire to teach their children about what had happened as well as an obligation to say a prayer here for those who perished.

Kor said she had not come back earlier because "after five years I barely knew who I was. After 20 years, I still couldn't go back. Maybe it takes 30 or 40 years for children who go through such experiences to cope with the pain and the memories."