Tens of thousands of people are expected to gather in the evening at the "ground zero" site in New York City to mark the 14th anniversary of the September 11, 2001, terrorist attacks in the United States.
The former site of the World Trade Center is now a memorial plaza and museum.
The anniversary will be marked with the tolling of bells and the reading of the names of the nearly 3,000 people who died when Al-Qaeda terrorists hijacked and crashed four passenger airliners.
Two of the planes crashed into the World Trade Center; one crashed into the Pentagon in Washington, D.C.; and the fourth crashed in a field in Pennsylvania after passengers overpowered the terrorists.
President Barack Obama will mark the occasion with a visit to the Fort Meade military base in Maryland, while Defense Secretary Ash Carter will join other officials for a memorial at the Pentagon.
Other events are scheduled across the country.