UNITED NATIONS ( CNN ) -- The 15th anniversary of the genocide in Rwanda brought American U.N . Ambassador Susan Rice to tears as she reflected on her personal memories surrounding the slaughter . U.N . Ambassador Susan Rice recalls her time in Rwanda working for the Clinton administration . Rice visited Rwanda as a staff member in former President Bill Clinton 's National Security Council six months after the ethnic cleansing . She explained how even months after the violence she encountered decomposing bodies at one of the massacre sites . `` For me , the memory of stepping around and over those corpses will remain the most searing reminder imaginable of what our work here must aim to prevent , '' she said Tuesday at a special commemorative event at United Nations headquarters in New York . She emotionally concluded her remarks , expressing how `` We bow our heads to mark the memory of those who were slain . And we bow our heads to mark the sorrow of all who stood by . '' The mass killings began on April 6 , 1994 , when tribal Hutu militia members attacked their tribal Tutsi countrymen after the plane of then-President Juvenal Habyarimana , a Hutu , was shot down . Within 100 days , some 800,000 people were murdered . In addition to Rice , various dignitaries , ambassadors , artists , U.N. officials and genocide survivors gathered Tuesday to mark the tragic circumstances that began 15 years ago . U.N. Secretary-General Ban Ki-moon reminded the audience how the Rwandan genocide continues to haunt `` our collective conscience '' and that `` the prevention of genocide is a collective responsibility . '' Ban encouraged the world community to remember its shared duty to prevent future catastrophes in the name of humanity and in honor of the memory of those who died . The United Nations created a Special Representative for the Prevention of Genocide and Mass Atrocities in 2004 . President Obama also released a statement coinciding with Tuesday 's anniversary . Obama praised survivors for their remarkable strength , but emphasized the tragedy should always be remembered because each victim `` had their own story , their own family , and their own dreams . ''
U.N . Ambassador Susan Rice recalls stepping over bodies in Rwanda . United Nations staff recalls 1994 mass killings in Rwanda . Obama says tragedy will always be remembered .