NEW YORK (AP) -- The 9/11 anniversary commemoration at ground zero began Saturday with a tolling bell and a moment of silence, exactly 20 years after the start of the deadliest terror attack on U.S. soil.
President Joe Biden, former presidents Barack Obama and Bill Clinton joined a crowd of victims' relatives and first responders at the Sept. 11 memorial plaza in New York. The memorial stands where the World Trade Center's twin towers were rammed and felled by hijacked planes.
Observances are also planned at the two other sites where the 9/11 conspirators crashed the jets -- the Pentagon and a field near Shanksville, Pennsylvania. Biden is scheduled to pay respects at all three places.
The anniversary comes under the pall of a pandemic and in the shadow of the U.S. withdrawal from Afghanistan, now ruled by the same militants who gave safe haven to the 9/11 plotters.
The attacks killed nearly 3,000 people and marked the start of a new era of fear, war, politics, patriotism and tragedy.
"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," said Thea Trinidad, who lost her father in the attacks and has signed up to read victims' names at the ceremony at ground zero in New York.
In a video released Friday night, Biden mourned the ongoing losses of 9/11.
"Children have grown up without parents, and parents have suffered without children," said Biden, a childhood friend of the father of a Sept. 11 victim, Davis Grier Sezna Jr.
But the president also spotlighted what he called the "central lesson" of Sept. 11: "that at our most vulnerable ... unity is our greatest strength."
Former President George W. Bush, the nation's leader on 9/11, is due at the Pennsylvania memorial and his successor, Barack Obama, at ground zero. The only other post-9/11 U.S. president, Donald Trump, plans to be in New York, in addition to providing 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 -- are planned across a country now full of 9/11 plaques, statues and commemorative gardens.
Using hijacked planes as missiles, the assailants inflicted the deadliest terrorist attacks on U.S. soil, taking nearly 3,000 lives, toppling the twin towers and ushering in an age of fear.
Security was redefined, with changes to airport checkpoints, police practices and the government's surveillance powers. In the years that followed, 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.
New York faced questions early on about whether it could ever recover from the blow to its financial hub and restore a feeling of safety among the crowds and skyscrapers. New Yorkers ultimately rebuilt a more populous and prosperous city but had to reckon with the tactics of an empowered post-9/11 police department and a widened gap between haves and have-nots.
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 concerned that al-Qaida, the terror network behind 9/11, may regroup in Afghanistan.
Two decades after helping to triage and treat injured colleagues at the Pentagon on Sept. 11, retired Army Col. Malcolm Bruce Westcott is saddened and frustrated by the continued threat of terrorism.
"I always felt that my generation, my military cohort, would take care of it -- we wouldn't pass it on to anybody else," said Westcott, of Greensboro, Georgia. "And we passed it on."
For Angelique Tung, who was at the trade center for a business meeting on 9/11 and escaped down 77 flights of stairs, the U.S. pullout from Afghanistan stirred empathy for troops who served there. Some now wonder whether their efforts and sacrifices made a difference, which makes Tung think of a question she has asked herself since surviving Sept. 11.
"I hope that, after 20 years, other people are asking that question: What good can come from this?" said Tung, of Wellesley, Massachusetts.
Sept. 11 propelled a surge of shared grief and common purpose, but it soon gave way.
Muslim Americans endured suspicion, surveillance and hate crimes. The quest to understand the catastrophic toll of the terror attacks prompted changes in building design and emergency communications, but it also spurred conspiracy theories that seeded a culture of skepticism. Schisms and resentments grew over immigration, the balance between tolerance and vigilance, the meaning of patriotism, the proper way to honor the dead, and the scope of a promise to "never forget."
Trinidad was 10 when she overheard her dad, Michael, saying goodbye to her mother by phone from the burning trade center. She remembers the pain but also the fellowship of the days that followed, when all of New York "felt like it was family."
"Now, when I feel like the world is so divided, I just wish that we can go back to that," said Trinidad, of Orlando, Florida. "I feel like it would have been such a different world if we had just been able to hang on to that feeling."