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Moore, Okla.- This aerial view shows Plaza Towers
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Moore, Okla.- This aerial view shows Plaza Towers Elementary School, which was destroyed in the deadly tornado. Unlike several others schools in the Oklahoma City area, Plaza Towers had no "safe room" in which students and teachers could huddle. (May 21, 2013)(Credit: AP )

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Most deadly U.S. tornadoes

Take a look at the most deadly U.S. tornadoes, dating back to the 1800s.