Emergency crews and volunteers continued to work through the early morning hours Tuesday in a frantic search for survivors of a huge tornado that ripped through parts of Oklahoma City and its suburbs, killing at least 91 people, 20 of them children, and flattening whatever was in its path, including a hospital and at least two schools. Much of the tornado damage appeared to be in the suburb of Moore, where rescue workers struggled to make their way through debris-clogged streets and around downed power lines to those who are feared trapped under mountains of rubble.