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Associated Press
In this undated image released by Mexico's National Institute of History and Anthropology, INAH, shows human blood cells seen through an electronic microscope, found in sharp obsidian knives discovered in Cantona, in the central Mexico state of Puebla. Researchers in Mexico announced Wednesday that they have found blood cells and fragments of muscle, tendon, skin and hair on 2,000-year-old stone knives, calling it the first conclusive evidence from a large number of stone implements pointing to their use in human sacrifice. (AP Photo/INAH/Luisa Mainou, HO)

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