The Tribute in Light, marking the 18th anniversary of the 9/11 attacks in New York, is seen from Manhattan, New York, the United States, Sept. 11, 2019. (Photos: Xinhua)

The Tribute in Light, marking the 18th anniversary of the 9/11 attacks in New York, is seen from Manhattan, New York, the United States, Sept. 11, 2019.

The Tribute in Light, marking the 18th anniversary of the 9/11 attacks in New York, is seen from Manhattan, New York, the United States, Sept. 11, 2019.

The Tribute in Light, marking the 18th anniversary of the 9/11 attacks in New York, is seen from Manhattan, New York, the United States, Sept. 11, 2019.

People attend an event to mourn the victims of the 9/11 terror attacks in Livermore, California, the United States, Sept. 11, 2019.

Shoe memorials are arranged to mourn the victims of the 9/11 terror attacks in Livermore, California, the United States, Sept. 11, 2019.

Shoe memorials are arranged to mourn the victims of the 9/11 terror attacks in Livermore, California, the United States, Sept. 11, 2019.

Shoe memorials are arranged to mourn the victims of the 9/11 terror attacks in Livermore, California, the United States, Sept. 11, 2019.