>>>The U.S. Holocaust Memorial Museum marked its 20th anniversary on Monday with what may be one of the last large gatherings of Holocaust survivors. Former U.S. President Bill Clinton urged the museum to recommit to preserving the memory of the Nazi genocide after all the survivors are gone.

They once were treated as less than human, but here these Holocaust survivors were heroes.

And that meant a lot to Tamara Wohl, who spent the first years of her life in a concentration camp. "I'm extremely proud, and very proud of all the work that the people of the museum did. I'm very grateful," she said.

The U.S. Holocaust Memorial Museum was dedicated in 1993 by then-President Bill Clinton and author and survivor Elie Wiesel.

Both returned on Monday for the museum's 20th Anniversary.  Wiesel recalled how at the time he admonished the president. "And I spoke to you about Yugoslavia - I had just come back from there - what we must do, in the name of our memory," she stated.

Clinton recalled that Wiesel and other Jews, haunted by the slaughter of six million of their own , pushed for intervention to stop the massacre of Muslims in Bosnia and Kosovo.

He said the Holocaust Museum is unlike Washington's other monuments.

"The Holocaust memorial will be our conscience, will be here as our conscience, from now, forever," Clinton said. "The museum must look to the future and make sure that as direct memories fade away, that the records, the pictures and the stories never die."

There are more than 800 Holocaust survivors here today and 130 American World War II veterans. But the day is approaching when the museum will have to tell the story of what happened without them.

Szoszana Schwarz was one of many survivors who came and donated some family documents to the museum.

Susan Bachrach is curator of a new exhibit on collaboration and complicity in the Holocaust. "An awful lot of people think, based on visitor testing - that it's all about Hitler," she said. "But it needed many helpers all over Europe."

Mike Abramowitz directs the museum's genocide prevention program. He says the exhibit also honors gentiles who risked their lives to save Jews, to teach a moral lesson. "What you do matters, that individuals have an agency to do good, or to do evil," he explained. "To be a bystander or to be a rescuer."

World War II veteran Alfred Henick understands why young people need to be reminded. "I remember when I was a kid in the 1930s in elementary school, and they used to teach us about World War I. To me it was ancient history," he stated. "World War I, it was only 15 years prior."

He said that's why this museum is needed, to ensure that the Holocaust is never forgotten.