U.S. national flags fly at half-mast at the Washington Monument in commemoration of the 19th anniversary of the 9/11 attacks in Washington D.C., the United States, on Sept. 11, 2020. People on Friday commemorated the 19th anniversary of the 9/11 attacks, which left nearly 3,000 killed or missing, and were the largest terrorist attacks on U.S. soil in history. (Xinhua/Liu Jie)