WASHINGTON, Sept. 11 (Xinhua) -- Former U.S. President Donald Trump was absent at official 9/11 memorial ceremonies in New York City and Shanksville, Pennsylvania, on Saturday.

President Joe Biden, former President Barack Obama and former President Bill Clinton all attended a somber ceremony Saturday morning at the National September 11 Memorial where the World Trade Center towers fell two decades ago.

Biden traveled next to the Flight 93 memorial in Shanksville to attend a wreath-laying ceremony. Earlier, former President George W. Bush and Vice President Kamala Harris both spoke at the memorial.

Trump, instead of appearing at the ceremonies, traveled several blocks from his Trump Tower building in Manhattan to the 17th police precinct and the neighboring fire station in NYC.

During an unannounced stop at a Shanksville fire department, Biden praised Bush for encouraging American unity in his speech. He also defended his administration's handling of the withdrawal from Afghanistan in brief remarks to the press pool.

Biden's last stop in the day was the National 9/11 Pentagon Memorial in Washington, D.C., where he attended a wreath-laying ceremony along with first lady Jill Biden, Vice President Kamala Harris, Defense Secretary Lloyd Austin and Gen. Mark Milley, chairman of the Joint Chiefs of Staff.

Biden did not deliver formal remarks on Saturday but released a video statement on Friday recognizing the lives lost in the deadliest attack in U.S. history and calling for national unity.

Nineteen terrorists hijacked four fuel-loaded U.S. commercial airplanes bound for west coast destinations on Sept. 11, 2001. A total of 2,977 people were killed in New York City, Washington, D.C. and outside of Shanksville, Pennsylvania.