Citizens pay tribute to victims of the sunken South Korean ferry "Sewol" at a group memorial altar at the Yellow Ribbon Park in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)

A woman writes down hope for safe return of missing passengers of the sunken South Korean ferry "Sewol" at a group memorial altar in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)

Citizens wait in line to pay tribute to victims of the sunken South Korean ferry "Sewol" at a group memorial altar in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)

Citizens pay tribute to victims of the sunken South Korean ferry "Sewol" at a group memorial altar in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)

Citizens pay tribute to victims of the sunken South Korean ferry "Sewol" at a group memorial altar in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)

A woman holding a white chrysanthemum waits to pay tribute to victims of the sunken South Korean ferry "Sewol" at a group memorial altar in Seoul, South Korea, on May 1, 2014. (Xinhua/Peng Qian)