The anniversary was marked by several events across the country.
Saturday marks 20 years since the 9/11 terrorist attacks.
Hijackers crashed two commercial airliners into the World Trade Center, striking the north tower at 8:46 a.m. followed by the south tower at 9:03 a.m. At 9:37 a.m., a third hijacked airline crashed into the Pentagon.
Twenty-two minutes later, the World Trade Center's south tower collapsed. A fourth hijacked plane crashed into a field in rural Shanksville, Pennsylvania, at 10:03 a.m. after passengers fought with and overcame the hijackers.
At 10:28 a.m. the World trade Center's north tower collapsed.
In total, 2,977 people were killed, including many New York City first responders.
The anniversary will be marked by several events across the country, including the annual commemoration at the World Trade Center Memorial in downtown Manhattan.
NATO marked the 20th anniversary of the Sept. 11 attacks on Saturday with a commemoration ceremony in Brussels.
Secretary General Jens Stoltenberg was joined by the U.S. Mission to NATO's Chargé d'Affaires Douglas D. Jones at the ceremony.
A wreath was laid and those in attendance fell silent as they paid their respects to those who died in the attacks
-ABC News' Guy Davies
A sixth moment of silence was held at the World Trade Center memorial at 10:28 a.m. to mark the time when the North Tower collapsed.
It was the final moment of silence of the day's ceremonies.
After bells tolled, "Hamilton" actor Chris Jackson performed "Never Alone."
A fifth moment of silence took place at 10:03 a.m., the time when Flight 93 crashed in Shanksville, Pennsylvania, after passengers fought with and overcame the hijackers.
Bells tolled at the memorial site near where the plane crashed as well as at the World Trade Center memorial.
A fourth moment of silence took place at 9:59 a.m., the time when the south tower of the World Trade Center collapsed.
Bells tolls at the at the National September 11th Memorial & Museum and at locations across the city to mark this moment.
