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NEW YORK - Twenty years ago, the 11th of September dawned as just a date. By midnight, it was 9/11, the staggering starting point for a new era of terror, war, politics, patriotism and tragedy.

The U.S. marks the milestone anniversary Saturday under the pall of a pandemic and in the shadow of a frantic withdrawal from Afghanistan, which fell to the same militant rulers who gave safe haven to the plotters of the 2001 attacks.

"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.

President Joe Biden is scheduled to travel to all three attack sites: New York's World Trade Center, the Pentagon and a field near Shanksville, Pennsylvania.

In a video released Friday night, he 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."

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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, is planning 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.

9/11 fallout includes decades of worry about toxic WTC dust

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.

MORE 9/11 COVERAGE

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."

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Honoring those lost on 9/11

Two decades after 9/11, the pain of those terrorist attacks is still raw, as families prepare to mark the 20th anniversary of the loss of their loved ones.

MORE SEPTEMBER 11 COVERAGE:

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