Former U.S. President Barack Obama (1st L, C), his wife Michelle Obama (2nd L, C), U.S. President Joe Biden (3rd L, C) and his wife Jill Biden (4th L, C) attend a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

A U.S. national flag is shown by a guard of honor during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

Relatives read victims' names during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

U.S. President Joe Biden (C-L) and his wife Jill Biden pay their respects to victims of the 9/11 attacks during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

Artists perform during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

A guard of honor salutes during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

Relatives read victims' names during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

Violinist Randall Goosby performs during a commemoration ceremony of the 20th anniversary of the 9/11 attacks in New York, the United States, on Sept. 11, 2021. The commemoration ceremony was held here on Saturday to mark the 20th anniversary of the 9/11 attacks. (Xinhua/Wang Ying)

A woman takes photos during a ceremony marking the 20th anniversary of 9/11 attacks, in Grapevine, Texas, the United States, on Sept. 11, 2021. (Photo by Dan Tian/Xinhua)

A firefighter salutes during a ceremony marking the 20th anniversary of 9/11 attacks, in Grapevine, Texas, the United States, on Sept. 11, 2021. (Photo by Dan Tian/Xinhua)

American Airlines crew members march to mark the 20th anniversary of 9/11 attacks, in Grapevine, Texas, the United States, on Sept. 11, 2021. (Photo by Dan Tian/Xinhua)

People attend a ceremony marking the 20th anniversary of 9/11 attacks, in Grapevine, Texas, the United States, on Sept. 11, 2021. (Photo by Dan Tian/Xinhua)

People observe a moment of silence before a memorial walk in Chicago, the United States, on Sept. 11, 2021. A memorial walk through Chicago was organized on Saturday in memory of the 20th anniversary of 9/11 attacks and lives lost in Afghanistan. (Photo by Christopher Dilts/Xinhua)

A firefighter takes part in a ceremony marking the 20th anniversary of 9/11 attacks, in Grapevine, Texas, the United States, on Sept. 11, 2021. (Photo by Dan Tian/Xinhua)

People take part in a memorial walk in Chicago, the United States, on Sept. 11, 2021. A memorial walk through Chicago was organized on Saturday in memory of the 20th anniversary of 9/11 attacks and lives lost in Afghanistan. (Photo by Christopher Dilts/Xinhua)

People observe a moment of silence before a memorial walk in Chicago, the United States, on Sept. 11, 2021. A memorial walk through Chicago was organized on Saturday in memory of the 20th anniversary of 9/11 attacks and lives lost in Afghanistan. (Photo by Christopher Dilts/Xinhua)

People prepare to participate in a memorial walk in Chicago, the United States, on Sept. 11, 2021. A memorial walk through Chicago was organized on Saturday in memory of the 20th anniversary of 9/11 attacks and lives lost in Afghanistan. (Photo by Christopher Dilts/Xinhua)

People observe a moment of silence before a memorial walk in Chicago, the United States, on Sept. 11, 2021. A memorial walk through Chicago was organized on Saturday in memory of the 20th anniversary of 9/11 attacks and lives lost in Afghanistan. (Photo by Christopher Dilts/Xinhua)

U.S. national flags fly at half-staff at the Washington Monument commemorating the 20th anniversary of the 9/11 attacks in Washington, D.C., the United States, on Sept. 11, 2021. (Xinhua/Liu Jie)

U.S. national flags fly at half-staff at the Washington Monument commemorating the 20th anniversary of the 9/11 attacks in Washington, D.C., the United States, on Sept. 11, 2021. (Xinhua/Liu Jie)

A U.S. national flag flies at half-staff at the White House commemorating the 20th anniversary of the 9/11 attacks in Washington, D.C., the United States, on Sept. 11, 2021. (Xinhua/Liu Jie)

U.S. national flags fly at half-staff at the Washington Monument commemorating the 20th anniversary of the 9/11 attacks in Washington, D.C., the United States, on Sept. 11, 2021. (Xinhua/Liu Jie)