A son of a 9/11 attacks' victim speaks at a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
Relatives of the victims of 9/11 attacks attend a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying) 
A national flag of the United Staes is displayed at a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A national flag of the United Staes is displayed at a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
U.S. Republican presidential candidate Donald Trump (center left) arrives at a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
U.S. Democratic presidential candidate Hillary Clinton (C) arrives at a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A woman cries at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A victim's picture is seen during a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A woman cries during a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A girl cries during a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A man holds a picture of his lost relative during a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
Two people hug each other during a ceremony marking the 15th anniversary of September 11 attacks at the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
A man pays tribute in front of a wall dedicated to lost firefighters near the National 9/11 Memorial, in New York, the United States, on Sept. 11, 2016. The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)
