The Tribute in Light installation, the World Trade Center, and the skyline of New York are seen ahead of the 20th anniversary of the September 11 attacks in Manhattan, as seen from Exchange Place, New Jersey, U.S., September 10, 2021. U.S. President Joe Biden on Friday called for national unity as the United States prepared to mark the 20th anniversary of the Sept. 11, 2001, attacks that killed 2,977 people, calling it America's greatest strength in the face of adversity. REUTERS/Eduardo Munoz

People view the Tribute in Light beams and the One World Trade Center tower from Little Island park on the eve of the 20th anniversary of the September 11 attacks in Manhattan, New York, U.S., September 10, 2021. On Saturday, Biden and first lady Jill Biden will visit New York City, where two airliners destroyed the World Trade Center and killed 2,753 people REUTERS/Mike Segar

A general view of the Tribute in Light installation and the World Trade Center from a Staten Island Ferry ahead of the 20th anniversary of the September 11 attacks in New York City, U.S., September 10, 2021. REUTERS/Mario Anzuoni

People take photos of the Tribute in Light installation and the World Trade Center from a Staten Island Ferry ahead of the 20th anniversary of the September 11 attacks in New York City, U.S., September 10, 2021. REUTERS/Mario Anzuoni

People take photos of the Tribute in Light installation and the World Trade Center from a Staten Island Ferry ahead of the 20th anniversary of the September 11 attacks in New York City, U.S., September 10, 2021. REUTERS/Mario Anzuoni

The Tribute in Light installation, the World Trade Center, and the skyline of New York as a man walks ahead of the 20th anniversary of the September 11 attacks in Manhattan, as it is seen from Exchange Place, New Jersey, U.S., September 10, 2021. REUTERS/Eduardo Munoz