The United States solemnly marked the 20th anniversary of 9/11 on Saturday less than two weeks after the end of the war in Afghanistan.

The ceremony at ground zero in New York began exactly two decades after the attack started with the first of four hijacked planes crashing into one of the World Trade Center’s twin towers.

“It felt like an evil specter had descended on our world, but it was also a time when many people acted above and beyond the ordinary,” said Mike Low, whose daughter, Sara Low, was a flight attendant on that plane.

Her family has “known unbearable sorrow and disbelief” in the years since, the father told a crowd that included President Joe Biden and former presidents Barack Obama and Bill Clinton.

But “as we carry these 20 years forward, I find sustenance in a continuing appreciation for all of those who rose to be more than ordinary people,” Low said.

The anniversary unfolded under the pall of a pandemic and in the shadow of the U.S. withdrawal from Afghanistan, which is now ruled by the same militants who gave safe haven to the 9/11 plotters.

“It’s hard because you hoped that this would just be a different time and a different world. But sometimes history starts to repeat itself and not in the best of ways,” Thea Trinidad, who lost her father in the attacks, said before reading victims’ names at the ceremony.

K and Broadway actor Kelli O’Hara sang at the commemoration, but by tradition, no politicians spoke there. In a video released Friday night, Biden addressed the continuing pain of loss but also spotlighted what he called the “central lesson” of Sept. 11: “that at our most vulnerable … unity is our greatest strength.”

Biden was also scheduled to pay respects at the two other sites where the 9/11 conspirators crashed the jets: the Pentagon and a field near Shanksville, Pennsylvania. Together, the attacks killed nearly 3,000 people.

Calvin Wilson came to the Pennsylvania memorial to reflect on his brother-in-law LeRoy Homer, the first officer of the plane that crashed in Pennsylvania after passengers and crew fought to regain control. Hijackers are believed to have been targeting the U.S. Capitol or the White House.

Wilson said he believes a polarized country has “missed the message” of the passengers’ and crew members’ heroism.

“We don’t focus on the damage. We don’t focus on the hate. We don’t focus on retaliation. We don’t focus on revenge,” Wilson said. “We focus on the good that all of our loved ones have done.”

Former President George W. Bush, the nation’s leader on 9/11, and current Vice President Kamala Harris were to speak at the Pennsylvania memorial. The only other post-9/11 U.S. president, Donald Trump, planned to be in New York, in addition to providing a commentary at a boxing match in Florida in the evening.

Other observances — from a wreath-laying in Portland, Maine, to a fire engine parade in Guam — were planned across a country now full of 9/11 plaques, statues and commemorative gardens.

Changes, conflict after 9/11

In the aftermath of the attacks, security was redefined, with changes to airport checkpoints, police practices and the government’s surveillance powers. For years afterward, virtually any sizeable explosion, crash or act of violence seemed to raise a dire question: “Is it terrorism?” Some ideological violence and plots did follow, though federal officials and the public have lately become increasingly concerned with threats from domestic extremists after years of focusing on international terror groups in the wake of 9/11.

A “war on terror” led to invasions of Iraq and Afghanistan, where the longest U.S. war ended last month with a hasty, massive airlift punctuated by a suicide bombing that killed 169 Afghans and 13 American service members and was attributed to a branch of the Islamic State extremist group. The U.S. is now concenered that al-Qaida, the terror network behind 9/11, may regroup in Afghanistan, where the flag of the Taliban militant group once again flew over the presidential palace on Saturday.