A bell is rung during the reading of the names of passengers and crew who died on Flight 93 during memorial services near the crash site of Flight 93 in Shanksville, Pa. Sunday Sept. 11, 2011.

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U.S. President Barack Obama and First Lady Michelle Obama walk across a field upon arrival by Marine One for a commemoration ceremony at the Flight 93 National Memorial on the 10th anniversary of the 9/11 attacks in Shanksville, Pa.

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President Barack Obama and first lady Michelle Obama place a wreath at the Flight 93 National Memorial Sept., 11, 2011, in Shanksville, Pa., on the 10th anniversary of the Sept. 11 attacks.

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President Barack Obama and first lady Michelle Obama pause for a moment of silence at the wreath laying ceremony at the Flight 93 National Memorial Sept. 11, 2011 in Shanksville, Pa., on the 10th anniversary of the Sept. 11 attacks.

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A bereaved family member mourns near the Wall of Names near the crash site of Flight 93 in Shanksville, Pa. Sunday Sept. 11, 2011.

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Visitors embrace in front of the Wall of Names near the crash site of Flight 93 in Shanksville, Pa. Sunday Sept. 11, 2011.

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U.S. military personnel bow their heads as they observe a moment of silence to mark the exact time when the first hijacked airplane crashed into the World Trade Center on Sept. 11, 2001, as they stand at Phase 1 of the permanent Flight 93 National Memorial, near the crash site of Flight 93 in Shanksville, Pa., Sunday, Sept. 11, 2011.

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People arrive to attend on Sept. 11, 2011 the 10th anniversary of the 9/11 attacks at the newly dedicated Flight 93 memorial at the site where the United plane crashed in Shanksville, Pa.

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Flowers are placed on the section of the Wall of Names dedicated to Captain Jason Dahl at the newly dedicated Flight 93 memorial at the site where the United plane crashed, marked by the boulder in the background, in Shanksville, Pa., on Sept. 10, 2011 as the U.S. prepares to mark the tenth anniversary of the Sept. 11, 2001 attacks.

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Left to right, former President George W. Bush, former first lady Laura Bush, former President Bill Clinton, Dr. Jill Biden and Vice President Joe Biden review the Wall of Names during the dedication of Phase 1 of the permanent Flight 93 National Memorial near the crash site of Flight 93 in Shanksville, Pa., on Sept. 10, 2011. The Wall of Names features the names of those passengers and crew aboard Flight 93 who died when the plane crashed on 9/11.

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U.S. President Barack Obama and First Lady Michelle Obama visit the crash site after attending in a commemoration ceremony at the Flight 93 National Memorial on the 10th anniversary of the 9/11 attacks in Shanksville, Pa. on on Sept. 11, 2011.

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The 17-ton boulder, left, that was placed to mark the site of the crash of Flight 93 at the Flight 93 Memorial in Shanksville, Pa., is shown on Aug. 1, 2011. In one sense, Shanksville is a tiny village far from major cities and cultural attractions. But the crash of Flight 93 in a field there on Sept. 11, 2001 has brought visitors from around the world to the national park site that now marks the spot where the hijacked plane came down.

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