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.
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.
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.
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
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.
