Of all the images the world saw of last week's disaster, among the most indelible were those of New York City firefighters. Dressed in heavy protective uniforms, their faces covered with ghostly ash, they searched for survivors in the rubble that had been the World Trade Center. As each long day and night blended into the next increasingly hopeless day, their persistence was a sad reminder of the colleagues who had gone before them - firefighters who had headed up stairwells and through smoke-filled hallways, telling people to get out and move as far from the buildings as possible, only to be trapped themselves when the skyscrapers fell.