Seventy years after the liberation of the Nazi concentration camps, the United States still lacks the laws needed to prosecute war criminals in its custody and give redress to victims.  This week, as world leaders mourn the victims of the Holocaust and commemorate the liberation of Buchenwald by U.S. forces, they should reflect on a key lesson: that mass atrocities must be met with justice.

On April 11, 1945, the American 6th Armored Division liberated Buchenwald, the largest Nazi concentration camp within Germany, where some 250,000 persons—Jews, Roma, Poles, and political and religious dissidents––were imprisoned.  At least 56,000 were murdered.  Weeks later, on April 29, 1945, U.S. forces liberated Dachau, Nazi Germany’s first concentration camp, where they found the now infamous sign: Arbeit Macht Frei.

But the liberators also found something else: evidence.  The ruins of the camps still stand today, testaments to a crime so staggering it needed new legal categories: crimes against humanity, genocide.  If the Holocaust was, as Winston Churchill declared, “the greatest and most horrible crime ever committed in the whole history of the world,” then such a crime demanded accountability.

Between 1945 and 1946, Nazi Germany’s major war criminals were tried and convicted by the International Military Tribunal at Nuremberg. Other trials followed, with the courts of France, Germany, Israel, and the United States prosecuting or deporting fugitive Nazi war criminals for years to come.  These trials had a restorative power for victims:  on the first day of trial at Nuremberg, Levi Shalitan, editor of the Yiddish newspaper Undzer Veg wrote:  “This trial shall be the last warning.  It must be proved that there is order and justice in our world. No one can again presume a world without justice and without judges.”

And yet 70 years after the liberation of the Nazi concentration camps, there are still war criminals who “presume a world without justice.”   By some estimates there are at least 1,000 suspected human rights abusers from 85 countries living in the United States who have committed torture, genocide or other serious human rights abuses.  At Nuremberg, a consensus emerged around a core principle: that certain crimes are so egregious they constitute crimes against all humanity and their perpetrators may be brought to justice wherever they are found.

Today, the Nuremberg principle is being challenged like never before.  In recent years, the international community has faced incontrovertible evidence of ongoing mass atrocities.  Tens of thousands have been tortured and killed in the military prisons of Syria and North Korea. Thousands of civilians have been slaughtered or enslaved by Boko Haram, ISIL, and other armed groups.

These distant atrocities are not so distant after all.  War criminals from today’s conflicts - Syria, Iraq, South Sudan, the Congo - are slipping into refugee flows, just as fugitive Nazi war criminals fled around the world after World War II.  Inevitably, these perpetrators will make their way to U.S. shores.  But we are unprepared.

Unlike our allies –– Canada, Britain, France –– the United States does not have a crimes against humanity law that would authorize the Department of Justice to bring criminal charges against perpetrators in U.S. custody.  Nor does its war crimes statute reach atrocities committed overseas by and against foreigners.

Without these laws, the United States was caught off guard when Bosnia and Rwanda erupted into massacres –– and perpetrators resurfaced within our borders.  Zeliko Boskovic, a member of the Bosnian Serb Army who participated in the 1995 Srebrenica massacre, was found living in Oregon.  Yet prosecutors could only charge this mass murderer with immigration fraud.  Similarly, Beatrice Munyenyezi, who participated in mass rape and murder in Rwanda, could only be charged for lying in her visa application.

Without these laws, we will once again be caught off guard by today’s war criminals.  Our prosecutors need the full legal arsenal in the hunt for war criminals.  To honor the 70th anniversary of the liberation of Auschwitz, Buchenwald, and Dachau, Congress should take up a crimes against humanity bill and ensure the United States will never again be a safe haven for human rights violators.

Gilmore is a staff attorney at the Center for Justice and Accountability, an international human rights organization.