Photos: US marks 20 years since 9/11, in shadow of Afghan war's end

A general view of the World Trade Center from the Top of The Rock Observation deck on the 70th floor of the 30 Rockefeller Center tower ahead of the 20th anniversary of the September 11 attacks.

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 US 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.'

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 US 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.

Using hijacked planes as missiles, the assailants inflicted the deadliest terrorist attacks on US 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?'

A beam of light is visible over the World War II Memorial as part of the Towers of Light Tribute near the Pentagon.

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 US 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 US is now concerned that al-Qaida, the terror network behind 9/11, may regroup in Afghanistan.

People hold candles as they attend a 9/11 commemoration religious ceremony at the Greek Orthodox St. Nicholas National Shrine adjacent to the 9/11 Memorial & Museum.

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.'

People gather on a parking rooftop to watch the 'Tribute in Light' installation.