MOORE, Okla. — 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. The risk of tornadoes throughout the region remained high Tuesday, according to the National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman.