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.
A US Flag adorns a name at the National September 11 Memorial site of the north tower at World Trade Center in New York, Sept. 8, 2021.
Angela Weiss/AFP via Getty Images
The anniversary will be marked by several events across the country, including the annual commemoration at the World Trade Center Memorial in downtown Manhattan.
Here's how the news was developing. All times Eastern.
Sep 11, 2021, 5:24 PM EDT
Biden attends wreath-laying ceremony at Pentagon
The White House commemorated the 20th anniversary of 9/11 with a wreath-laying ceremony at the Pentagon Memorial, which honors the 184 people who perished in the attack on the military headquarters.
President Joe Biden and first lady Jill Biden lay a wreath at the Pentagon on the 20th anniversary of the September 11, 2001 attacks in Arlington, Va., Sept. 11, 2021.
Tom Brenner/Reuters
President Joe Biden, first lady Jill Biden, Vice President Kamala Harris and second gentleman Douglas Emhoff attended the ceremony, holding hands in front of the wreath while a trumpeter played "Taps." They were joined by Secretary of Defense Lloyd Austin and Joint Chiefs of Staff Chairman Gen. Mark Milley.
President Joe Biden and First Lady Jill Biden followed by Vice President Kamala Harris and Second Gentlemen Doug Emhoff, attend a wreath laying ceremony as they pay their respects to 9/11 victims at the Pentagon, Va., Sept. 11, 2021.
Brendan Smialowski/AFP via Getty Images
The memorial, which opened in 2008, is located just feet from where the hijacked American Airlines Flight 77 struck the west wall of the Pentagon.
The wreath-laying ceremony was Biden's final scheduled event to mark the 20th anniversary of 9/11, following visits to Shanksville, Pennsylvania, and New York City.
2:09
President and first lady attend wreath-laying ceremony at Pentagon
President Joe Biden, first lady Jill Biden, Vice President Kamala Harris and second gentleman Douglas Emhoff participated in the ceremony to lay a wreath at the Pentagon.
Sep 11, 2021, 4:49 PM EDT
Biden visits with Shanksville Volunteer Fire Department
President Joe Biden made an unannounced stop Saturday afternoon at the Shanksville Volunteer Fire Department -- the first to respond to the United Flight 93 crash in Shanksville, Pennsylvania, on Sept. 11, 2001.
The president spent nearly an hour taking photographs and speaking with over 100 first responders and their families.
President Joe Biden and first lady Jill Biden visit a cross made of steel from the north tower of the Word Trade Center near the Shanksville Volunteer Fire Department in Shanksville, Pa., Sept. 11, 2021. The Bidens stopped by after visiting the nearby Flight 93 National Memorial to commemorate the 20th anniversary of the Sept. 11, 2001, terrorist attacks.
Evan Vucci/AP
After his visit, Biden spoke briefly with reporters, where he reflected on the importance of memorials, like the ones erected in Shanksville to commemorate the victims of the 9/11 attacks.
“These memorials are really important, but they're also incredibly difficult for the people who were affected by them because it brings back that moment you got the phone call. It brings back that instant you got the news, no matter how many years go by," he said.
Unprompted, Biden said he thought former President George W. Bush gave a "really good speech" Saturday about about who Americans are.
"The core of who we are is not divided," Biden said.
When asked how the nation can get back to that state of unity, Biden said, “By being honest when I make a mistake. By being straightforward, telling people exactly what I want to do.”
1:24
Biden visits Shanksville Fire Department
President Joe Biden paid a visit to the Shanksville Volunteer Fire Company in Pennsylvania, which was first to respond to the Flight 93 crash on Sept. 11, 2001.
-ABC News' Justin Gomez
Sep 11, 2021, 1:11 PM EDT
World Trade Center reading of the names concludes
The families of victims of both the Sept. 11 attacks and the 1993 bombing at the World Trade Center concluded their annual reading of the names.
The ceremony concluded with a performance of "Taps."
The Tribute in Light installation and the World Trade Center is seen from the Top of The Rock Observation deck on the 70th floor of the 30 Rockefeller Center tower ahead of the 20th anniversary of the September 11 attacks in New York, Sept. 10, 2021.
Carlos Barria/Reuters
At sundown, the annual “Tribute in Light” will once again illuminate the sky in commemoration of the anniversary.
Sep 11, 2021, 12:56 PM EDT
Biden, first lady pay respects at Shanksville
President Joe Biden and first lady Jill Biden arrived at the Sept. 11 memorial services at Shanksville, Pennsylvania.
The president laid a wreath at the Flight 93 National Memorial and silently reflected in front of the wall with the names of the 40 victims.
President Joe Biden and First Lady Jill Biden lay a wreath at the Flight 93 National Memorial, marking the 20th anniversary of the 9/11 attacks in Shanksville, Pa., Sept. 11, 2021.
Jim Watson/AFP via Getty Images
The Bidens walked the grounds of the memorial and spoke with family members of the victims who were in attendance.