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Bill Clinton said the Bosnian Serb leaders responsible for the massacre must be found

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Thousands of victims' relatives gathered for the ceremony

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A choir in traditional Islamic dress sang a solemn tribute called Srebrenica Inferno

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Muslim dignitaries oversaw the ceremony as coffins were interred

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Some families lost sons as well as fathers

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More than 7,000 Muslims were massacred by General Ratko Mladic's forces