The sidewalks and streets surrounding flattened buildings were strewn with mangled cars, demolished rescue equipment and personal belongings. Thick layers of dust and paper that rained down after the explosion covered dozens of blocks. The Federal Emergency Management Agency, or FEMA, says 450,000 tons of debris would have to be cleared from the Twin Towers, and another 15,000 from the third building that collapsed. Mayor Giuliani said 3,000 tons of debris had already been removed.