Updated September 11, 2021 at 5:34 PM ET
Twenty years to the day after a pair of hijacked airliners destroyed the World Trade Center towers and another plane punched a gaping hole in the Pentagon and a fourth passenger jet crashed in a Pennsylvania field after passengers sought to regain control from hijackers, Americans nationwide reflected on the events that forever changed their country.Nearly 3,000 people were killed on Sept. 11, 2001. The event not only sparked enormously costly and largely unwinnable wars in both Afghanistan and Iraq, but also spawned a domestic war on terrorism, rewriting the rules on security and surveillance in the U.S., the repercussions of which continue to reverberate.To commemorate the day, hundreds of people on Saturday gathered in Lower Manhattan at the National September 11 Memorial & Museum on the spot where the World Trade Center's twin towers once stood. Three presidents — President Biden, former Presidents Barack Obama and Bill Clinton — and their wives attended. They wore blue ribbons and held their hands over their hearts as a procession marched a flag through the memorial and stood somberly side by side as the names of the dead were read off by family members and stories and remembrances were shared.The president and first lady also met with former New York Mayor Michael Bloomberg and his partner, Diana Taylor, according to the White House. They greeted FBI Director Christopher Wray, House Speaker Nancy Pelosi, the New York congressional delegation, and many other current and former state and local officials as they arrived at the memorial. Rudy Giuliani, the mayor of New York City at the time of the attacks, also attended the ceremony.At a ceremony at Shanksville, Pa., former President George W. Bush remembered the day that "the world was loud with carnage and sirens. And then silent with voices." Bush lamented the current era of political division, seemingly alluding to the Jan. 6 insurrection at the U.S. Capitol. "We have seen growing evidence that the dangers to our country can come, not only across borders, but from violence that gathers within," Bush said. "There is little cultural overlap between violent extremists abroad and violent extremists at home ... [but] they are children of the same foul spirit, and it is our continuing duty to confront them."Also in Shanksville, where a hijacked plane crashed after passengers fought back, Vice President Harris called the site "hallowed ground."United Flight 93 taught us "about the courage of those on board, who gave everything. About the resolve of the first responders, who risked everything. About the resilience of the American people," she said.Echoing Bush, Harris said that in the days after the attacks, "we were all reminded that unity is possible in America. We were reminded, too, that unity is imperative in America. It is essential to our shared prosperity, our national security, and to our standing in the world." At ground zero in New York City, the national anthem was performed in a solemn ceremony, and then, in what has become an annual tradition, a moment of silence was observed at 8:46 a.m., when American Airlines Flight 11 crashed into the north tower.The names of the victims were read allowed by family members, who shared anecdotes and remembrances of their loved ones.Another moment of silence was observed at 9:03 a.m., when United Flight 175 hit the south tower, 9:59 a.m., when the south tower collapsed, and 10:28 a.m., when the north tower of the World Trade Center came down.See more NPR coverage of the 20th anniversary of the Sept. 11 attacks.More than 2,600 people were killed in and around the World Trade Center buildings. At the Pentagon, 184 died, and 40 more were killed in Pennsylvania.Among those who attended the ceremony in Manhattan was Bruce Springsteen, who with an acoustic guitar and harmonica, took the dais to perform "I'll See You In My Dreams." The New York Police Department pipes and drums band also played "Hard Times Come Again No More," a U.S. folk song dating from the 1850s.Biden made no remarks on Saturday in New York, but speaking on Friday, he said that in the days after the attacks in 2001, "we saw heroism everywhere — in places expected and unexpected.""We also saw something all too rare: a true sense of national unity," the president said.A moment of silence was also observed at 9:37 a.m., marking when American Airlines Flight 77 careened into the west face of the Pentagon. A ceremony there was hosted by Secretary of Defense Lloyd Austin and Chairman of the Joint Chiefs of Staff Army Gen. Mark Milley.The Bidens also attended a wreath-laying ceremony at Shanksville, and another later in the day at the Pentagon in northern Virginia. At the Pentagon, linked hand to hand, the Bidens, Harris and second gentleman Doug Emhoff bowed their heads as they observed a moment of silence. Meanwhile, former President Donald Trump released a video message Saturday morning, largely lambasting Biden's handling of the withdrawal from Afghanistan. Trump, who visited Shanksville on Friday, visited a police precinct and fire department in New York City on Saturday, and is scheduled to deliver ringside commentary at a boxing match at a casino in Hollywood, Fla.In London, acting ambassador to the United Kingdom Philip Reeker attended a special changing of the guard at Windsor Castle, at which the U.S. national anthem was performed. Reeker said Americans would be "forever grateful" for the "enduring friendship" between the two countries.The 20th anniversary of the attacks comes just weeks after the chaotic final withdrawal of U.S. forces from Afghanistan, ending America's longest war.Following the 2001 attacks, then-President Bush ordered "boots on the ground" in Taliban-controlled Afghanistan to root out al-Qaida and hunt for the mastermind of the attacks, Osama bin Laden. The war passed to his successor, Obama. Under Obama's watch, bin Laden was located in Pakistan and killed in a covert U.S. military operation. But the war dragged on. The Trump White House negotiated directly with the Taliban for a complete withdrawal of U.S. forces, which was completed last month.However, as U.S. troops were leaving, the Taliban were also gaining the upper hand against American-trained Afghan security forces, resulting in the quick collapse of the Afghan government.Some families of the victims of 9/11 had asked Biden not to attend the 20th anniversary memorial events unless he ordered the declassification of documents they say will show that Saudi Arabian leaders lent material support to bin Laden.During his campaign, Biden had promised that if elected, he would direct the Justice Department to release more information related to the attacks in a "narrowly tailored" way."The 9/11 families are right to seek full truth and accountability," he said.Earlier this month, Biden signed an executive order to begin declassifying those documents. Copyright 2021 NPR. To see more, visit https://www.npr.org.