UNITED NATIONS - Twenty years ago, the killers roved about Rwanda in machete-wielding bands from village to village, house to house, lopping off the limbs of neighbors, friends and relatives -- anyone who was a Tutsi or sympathized with them.

On April 7, 1994, the small central African country with a population of about 7 million people convulsed in a 100-day spasm of horrific violence as the world watched 800,000 to 1 million people hacked to death in homes, houses of worship, sports stadiums. Rape was rampant.

On Monday, the UN marked the 20th anniversary of the genocidal campaign waged by Hutus against Tutsis, with UN Secretary-General Ban Ki-moon calling it "one of the darkest chapters in human history."

Ban joined Rwandans and the ambassadors of several member states on the UN Security Council in Kigali, Rwanda's capital, to commemorate the slaughter, recognize the UN personnel killed during the attacks and to acknowledge that it might not have happened had the international community, including the United Nations, done more.

"The blood spilled for 100 days," Ban said. "Twenty years later, the tears still flow. I express my solidarity with all Rwandans as you continue your journey of healing. I also recognize the devastating consequences that the wider region continues to feel."

He added: "Many United Nations personnel and others showed remarkable bravery. But we could have done much more. We should have done much more."

On April 6, 1994, a plane carrying Rwanda's president Juvenal Habyarimana was shot down as he returned from a peace conference, killing him and Burundi's president, Cyprian Ntayamira -- an event that touched off the wave of violence against Tutsis and their Hutu sympathizers. Two weeks into the surge of violence, UN member states asked to have their troops withdrawn, reducing the UN force from 2,165 troops to 270, according to UN reports.

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A 2000 UN report concluded that "Genocide in Rwanda went as far as it did in part because the international community failed to use or to reinforce the operation then on the ground in that country to oppose obvious evil."

Samantha Power, the U.S. ambassador to the UN, also led a delegation to the commemoration Monday.

"I have come to Rwanda to join with Rwandans and people from all around the world to express our enduring grief and sadness at the momentous crime that transpired in this country two decades ago," she said. "The Rwandan genocide is a devastating reminder that nightmares seemingly beyond imagination can, in fact, take place."

UN General Assembly President John Ashe said in a statement: "The 100 days of unspeakable violence that took place in 1994 and the failure of the international community to respond to the cries of the many victims of this atrocity still haunts the world today."

Ban said the massacre helped establish in 2000 the UN Security Council's Responsibility to Protect resolution, adopting a doctrine that prevents states from claiming "that atrocity crimes are only a domestic matter" and empowers the United Nations to intervene to restore peace.

Dozens of participants and leaders of the massacre have been prosecuted through the UN's International Criminal Tribunal for Rwanda. The court has indicted 93 people and sentenced 61 of them, according to UN reports. Fourteen people have been acquitted.

"International criminal justice is expanding its reach," Ban said. "Leaders and warlords alike face the growing likelihood of prosecution for their crimes. The remarkable work of the International Criminal Tribunal for Rwanda has shown once again how justice is indispensable for sustainable peace."