Law enforcement authorities in New York and Washington were on high alert against what was described as a "credible but unconfirmed" threat of an al Qaeda plot to attack the United States again a decade after the toppling of the World Trade Center's twin towers by hijacked airliners.
Security was especially tight in Manhattan, where police set up vehicle checks on city streets as well as bridges and tunnels coming into the city.
President Barack Obama and former President George W. Bush, who was president at the time of the attacks, were scheduled to be among the dignitaries at the Ground Zero site in New York.
They were set to join victims' families to hear the reading of the names of those who died on September 11. Bells will toll across the city.
In the attacks, 19 men from the Islamic militant group al Qaeda hijacked airliners and crashed them into the World Trade Center, the Pentagon outside Washington and a field in Shanksville, Pennsylvania.
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