(NEW YORK) -- 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.
Here's how the news was developing. All times Eastern.
Sep 11, 11:38 am
Harris reflects on the unity in America following attacks
Vice President Kamala Harris spoke at the Sept. 11 memorial service at Shanksville, Pennsylvania and reflected on the heroism of the passengers who fought the hijackers.
"What happened on Flight 93 told us then, and it still tells us so much about the courage of those on board who gave everything they possibly could, about the resolve of the first responders who risked everything and about the resilience of the American people," she said.
Harris said Sept. 11, 2001 showed the power of unity among strangers and that sentiment is still powerful 20 years later. The vice president called on Americans to continue that camaraderie.
"In a time of outright terror, we turned toward each other. In the face of a stranger, we saw a neighbor and a friend. That time reminded us the significance and the strength of our unity as Americans, and that it is possible in America," she said.
Sep 11, 11:28 am
George W. Bush speaks at Shanksville, warns about domestic terrorism
Former President George W. Bush spoke at the Sept. 11 memorial at Shanksville, Pennslyvania, where United Flight 93 crashed after passengers fought with and overcame the hijackers.
Bush reflected on the bravery of those passengers amid the carnage taking place throughout the country.
"Here, the intended targets became the instruments of rescue, and many who are now alive owe a vast, unconscious debt to the defiance displayed in the skies above this field," he said.
Bush spoke about the unity and heroism that took place in the days after the attacks, the sights of people grabbing another person's hand and "rally to the cause of one another."
He also warned about the still lingering threat of terrorism, both foreign and domestic.
Bush said there is "little cultural overlap between violent extremists abroad and violent extremists at home.
"They are children of the same foul spirit and it's our continuing duty to confront them," he said.
Bush also commended the country's veterans and armed service members who have served since the attacks.
"Nothing that has followed, nothing, can tarnish your honor or diminish your accomplishments. To you and the honored dead, our country is forever grateful," he said.
Sep 11, 10:40 am
NATO marks 20th anniversary with memorial ceremony
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
Sep 11, 10:35 am
6th, final moment of silence held
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."
Sep 11, 10:08 am
5th moment of silence observed
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.
Sep 11, 10:03 am
4th moment of silence observed
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.
Sep 11, 9:56 am
Defense Secretary Lloyd Austin honors lives lost at Pentagon
Defense Secretary Lloyd Austin spoke at the Sept. 11 memorial at the Pentagon, honoring the 184 people killed in the building and aboard Flight 77.
"Today, we remember not just who our fallen teammates were, but the mission that they shared. And we recall their common commitment to defend our republic, and to squarely face new dangers," he said.
Austin talked about the members of the military who were born after the attacks and the sacrifices they've made to keep the nation safe, including the 13 service members who were killed in Afghanistan last month.
"It is our responsibility to remember, and it is our duty to defend our democracy," he said.
-ABC News' Matt Seyler
Sep 11, 9:47 am
3rd moment of silence held
At 9:37 a.m. a third moment of silence was held to mark the time when Flight 77 struck the Pentagon.
"Taps" played outside the Pentagon, while at the World Trade Center, a bell tolled.
Sep 11, 9:36 am
World leaders send their condolences on 20th anniversary
Leaders from around the world shared their reflections and condolences on the 20th anniversary of the Sept. 11 attacks.
Queen Elizabeth said in a statement she was still moved by the unity shown by the world during such a tragic time.
"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild," she said in a statement.
The U.S. national anthem played at the changing of the guard at Windsor Castle.
Ursula von der Leyen, the president of the European Union Commission, tweeted, "The EU stands with the USA in defending freedom & compassion over hate."
UN Secretary General Antonio Guterres issued a statement honoring first responders and that his "thoughts are with the victims and their families."
-ABC News' Guy Davies
Sep 11, 9:29 am
Bruce Springsteen performs solemn tribute
Bruce Springsteen performed following the second moment of silence at the World Trade Center memorial service.
His song "I'll See you in My Dreams," was dedicated to all of the lives lost that day.
Sep 11, 9:12 am
2nd moment of silence held
A second moment of silence was held at 9:03 a.m. to mark the time the south tower was hit.
A bell tolled at the World Trade Center memorial as well as other locations across the country.
Sep 11, 9:04 am
Families begin reading of names lost at World Trade Center
The families of the World Trade Center victims began their readings of the lives lost on Sept. 11.
Mike Low, whose daughter Sara was a flight attendant on Flight 11, spoke before the readings began. While he said his family's pain is still raw, they have found comfort in the community and outpouring of support from all over the world.
"As we recite the names we lost my memory goes back to that terrible day when it felt like an evil specter descended on our world, but it was also a time when people went above and beyond the ordinary," Low said.
Sep 11, 8:51 am
1st moment of silence held at World Trade Center
The memorial services at the National September 11th Memorial & Museum began with a color guard, bagpipes and a singing of the National Anthem.
The first moment of silence took place at 8:46 a.m., the time when the first plane struck the north tower.
A bell tolled to mark the moment and was echoed by bells across the city.
Sep 11, 9:15 am
Former presidents join Biden at World Trade Center
Former Presidents Barack Obama and Bill Clinton, former Secretary of State Hillary Clinton and former first lady Michelle Obama joined President Biden and first lady Jill Biden at the National September 11th Memorial & Museum ceremonies.
Former President George W. Bush is scheduled to meet with families in Shanksville, Pennsylvania.
Sep 11, 8:36 am
Crowds begin to gather at World Trade Center
Families of World Trade Center victims, survivors, first responders and dignitaries have begun gathering at the National September 11th Memorial & Museum for this year's memorial services.
The event begins at 8:30 a.m. and is expected to conclude at approximately 1:00 p.m.
The ceremony will include moments of silence at the times the four planes crashed and the times both towers collapsed.
Family members will read the names throughout the morning of all of the people lost that day as well as the victims of the 1993 World Trade Center bombing.
Sep 11, 8:01 am
Obama reflects on 20th anniversary
Former President Barack Obama released a statement Saturday morning reflecting on the 20th anniversary of the Sept. 11 attacks.
He urged all Americans to remember the courage and selflessness of those lost in the attacks.
"We reaffirm our commitment to keep a sacred trust with their families — including the children who lost parents, and who have demonstrated such extraordinary resilience. But this anniversary is also about reflecting on what we’ve learned in the 20 years since that awful morning," Obama wrote.
"That list of lessons is long and growing. But one thing that became clear on 9/11 – and has been clear ever since – is that America has always been home to heroes who run towards danger in order to do what is right."
In his statement the former president pointed to examples of heroic actions from the last two decades, such as the service members, first responders and medical personnel.
"They represent what is best in America, and what can and should bring us together," Obama said. "9/11 reminded us how so many Americans give of themselves in extraordinary ways – not just in moments of great crisis, but every single day. Let’s never forget that, and let’s never take them for granted."
-ABC News' Molly Nagle
Sep 11, 8:20 am
President Biden to attend services at World Trade Center, Shanksville, Pentagon
President Joe Biden and first lady Jill Biden will attend three 9-11 memorial services throughout the day
They will begin at the National September 11th Memorial & Museum in lower Manhattan at 8:30 a.m. and fly out to Shanksville, Pennsylvania. The president and first lady will attend a wreath-laying ceremony at the Flight 93 National Memorial at 12:30 p.m.
They will be joined by Vice President Kamala Harris and the second gentleman Doug Emhoff at the Pentagon at 4:30 p.m. where they will take part in the wreath-laying ceremony to honor the lives lost at the location.
Biden released a video speech on social media Friday evening marking the 20th anniversary.
"As we saw in the days that followed, unity is our greatest strength. It’s what makes us who we are — and we can’t forget that," he tweeted.
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