The Tribute in Light shined over lower Manhattan and the spot where the twin towers stood on the 20th anniversary of the terror attacks.

The lights, which were first installed as a monthlong tribute six months after the attacks, then lit up the sky on the second anniversary of Sept. 11. They have been used as a tribute each year on the anniversary since 2003. In 2020, the tribute was briefly canceled due to the coronavirus pandemic, but then later allowed to go on.

The blue lights are installed in Battery Park, just south of the World Trade Center.

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, 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.

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.

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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.

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.

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.”

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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

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."

At sundown, the annual “Tribute in Light” will once again illuminate the sky in commemoration of the anniversary.