A bullet is seen on a tomb at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Veterans mourn their friends at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Christian Jacobs lies besides the tomb of his father Christopher Jacobs at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Children stand near tombs at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Phoebe Quin mourns her brother Michael Quin at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Sandy Dyer mourns in front of the tomb of her son Scott Dyer at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Christian Jacobs stands besides the tomb of his father Christopher Jacobs at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Jared Pudvan mourns his friend Joe Wrightsman at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Mr. and Mrs. Cappelaere stands in front of the tomb of their daughter Valerie at section 60 of Arlington National Cemetery, outside Washington D.C., the United States, May 25, 2015. People crowded in Arlington National Cemetery, especially in Section 60, where those who died in America's most recent wars lie, during the Memorial Day.(Xinhua/Yin Bogu)

Members of US honour guard participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)

World War II veterans participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)

People participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)

Members of US honour guard participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)

People participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)

Members of US honour guard participate in a Memorial Day parade on Constitution Avenue in Washington D.C., the United States, on May 25, 2015. The Memorial Day is a United States federal holiday observed on the last Monday of May. (Xinhua/Yin Bogu)