Former President George W. Bush speaks at the Flight 93 National Memorial in Shanksville, Pa., Saturday, Sept. 11, 2021, on the 20th anniversary of the Sept. 11, 2001 attacks. (AP )

ARLINGTON, VIRGINIA - SEPTEMBER 11: (L-R) Second gentleman Doug Emhoff, U.S. Vice President Kamala Harris, first lady Jill Biden and U.S. President Joe Biden, participate in a wreath-laying ceremony on September 11, 2021 in Arlington, Virginia. The nation is marking the 20th anniversary of the terror attacks of September 11, 2001, when the terrorist group al-Qaeda flew hijacked airplanes into the World Trade Center, Shanksville, PA and the Pentagon, killing nearly 3,000 people. Kevin Dietsch/Getty Images/AFP == FOR NEWSPAPERS, INTERNET, TELCOS & TELEVISION USE ONLY ==

People watch the commemoration of the 20th anniversary of the Sept. 11 attacks at the Richard J. Daley Plaza in the Loop in Chicago, Saturday morning, Sept. 11, 2021. (AP)

"Taps" is played during the commemoration of the 20th anniversary of the Sept. 11 attacks at the Richard J. Daley Plaza in the Loop in Chicago, Saturday morning, Sept. 11, 2021. (AP)

President of A-Z Medical Services Corp Salman Aftab, from left, Deputy Mayor of Public Safety John O'Malley, first lady of Chicago Mayor Amy Eshleman, Chicago Mayor Lori Lightfoot and Chicago Fire Department Commissioner Annette Nance-Holt stand onstage during the commemoration of the 20th anniversary of the Sept. 11 attacks at the Richard J. Daley Plaza in the Loop in Chicago, Saturday morning, Sept. 11, 2021. ( AP)

TOPSHOT - A displays show a message during the commemoration of 20th anniversary of the 9/11 attacks in Times Square in New York on September 11, 2021. - America marked the 20th anniversary of 9/11 Saturday with solemn ceremonies given added poignancy by the recent chaotic withdrawal of troops from Afghanistan and return to power of the Taliban. At the 9/11 memorial in New York, a moment's silence was held at 8:46 am (1246 GMT), the time the first hijacked plane crashed into the World Trade Center on September 11, 2001. (AFP)

Chicago Police Department recruits walk past two fire trucks hoisting an American flag during the commemoration of the 20th anniversary of the Sept. 11 attacks at the Richard J. Daley Plaza in the Loop in Chicago, Saturday morning, Sept. 11, 2021. (AP)

Chicago Mayor Lori Lightfoot and first lady of Chicago Mayor Amy Eshleman have a moment of silence before offering flowers at the temporary 9/11 memorial during the commemoration of the 20th anniversary of the Sept. 11 attacks at the Richard J. Daley Plaza in the Loop in Chicago, Saturday morning, Sept. 11, 2021. ( AP)