Official commemorations are under way to commemorate 20 years since the 9/11 attacks against the US.
The ceremony in New York started with a minute's silence at 08:46 EST (13:46 BST) - the exact time the first plane hit the North Tower of the World Trade Center in 2001.
Thousands of people across the US have gathered to mark the sombre occasion.
President Joe Biden will travel to all three attack sites on Saturday - New York, Pennsylvania and Virginia.
A minute's silence also marked the moment the second plane crashed into the South Tower of the World Trade Center. Similar tributes will take place at the times when a third jet struck the Pentagon in Virginia, a fourth hijacked plane crashed into a field in Pennsylvania, and finally when the World Trade Center towers collapsed.