Events are taking place at all three sites directly affected by the hijacked planes on Sept. 11, 2001, as the U.S. commemorates the 20th anniversary of the terror attacks.

At Ground Zero in New York, President Biden joined former Presidents Bill Clinton and Barack Obama for a ceremony, which included six separate moments of silence. The first one took place at 8:46 a.m. ET, the time when the first plane struck the north tower of the World Trade Center complex. The bells rang again 17 minutes later — the second plane hit the south tower at 9:03 a.m. ET. The next four moments of silence then came at 9:37 a.m., 9:59 a.m, 10:03 a.m. and, finally, 10:28 a.m. to mark, respectively, when the third plane struck the Pentagon, the south tower collapsed, Flight 93 crashed into a Pennsylvania field after passengers revolted against the hijackers, and the north tower fell.

Meanwhile, former President George W. Bush (who was in office at the time) and Vice President Kamala Harris gave remarks at the Flight 93 National Memorial near Shanksville, Pennsylvania, where Biden is scheduled to arrive shortly. Defense Secretary Lloyd Austin addressed a crowd at the Pentagon.