NEW YORK — Three presidents and their wives stood somberly side by side at the National September 11 Memorial, sharing a moment of silence to mark the anniversary of the nation's worst terror attack with a display of unity.

Presidents Joe Biden, Barack Obama and Bill Clinton all gathered at the site where the World Trade Center towers fell two decades ago. They each wore blue ribbons and held their hands over their hearts as a procession marched a flag through the memorial, watched by hundreds of Americans gathered for the remembrance, some carrying photos of loved ones lost in the attacks. 

Before the event began, a jet flew overhead in an eerie echo of the attacks, drawing a glance from Biden toward the sky.

Biden was a senator when hijackers commandeered four planes and executed the attack. Now he marks the 9/11 anniversary for the first time as commander in chief.

The president will spend Saturday paying his respects at the trio of sites where the planes crashed, but he left the speech-making to others.

Instead, the White House released a taped address late Friday in which Biden spoke of the "true sense of national unity" that emerged after the attacks, seen in "heroism everywhere — in places expected and unexpected."

"To me that's the central lesson of September 11," he said. "Unity is our greatest strength."

Biden arrived in New York on Friday night as the skyline was illuminated by the "Tribute in Light," hauntingly marking where the towers once stood. 

Following the morning ceremony in New York City, Biden will visit the field near Shanksville, Pennsylvania, where a plane fell from the sky after heroic passengers fought terrorists to prevent it from reaching its Washington destination. And finally, he will head to the Pentagon, where the world's mightiest military suffered an unthinkable blow to its very home.

Biden's task, like his predecessors before him, was to mark the moment with a mix of grief and resolve. A man who has suffered immense personal tragedy, Biden speaks of loss with power.

He gave voice to the pain that comes with memories of 9/11 in his video message, saying, "No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago."

Firefighters stand beside the rim of the south pool adorned with flowers and flags to honor the fallen during ceremonies to commemorate the 20th anniversary of the Sept. 11 terrorist attacks, Saturday, Sept. 11, 2021, at the National September 11 Memorial & Museum in New York. (AP Photo/John Minchillo)

Robert Gibbs, who served as Obama's press secretary, said for Biden, "It's a moment for people to see him not as Democratic president, but as president of the United States of America,."

"The American people are somewhat conflicted about what they have seen out of Afghanistan the last couple of weeks," Gibbs said. "For Biden, it's a moment to try to reset some of that. Remind people of what it is to be commander in chief and what it means to be the leader of the country at a moment of such significance."

On the 20th anniversary of the attacks, Biden now shoulders the responsibility borne by his predecessors to prevent future tragedy, and must do so against fresh fears of a rise in terror after the United States' hasty exit from Afghanistan, the country from which the Sept. 11 attacks were plotted. 

Biden is the fourth president to console the nation on the anniversary of that dark day, one that has shaped many of the most consequential domestic and foreign policy decisions made by the chief executives over the past two decades. 

The terror attack defined the presidency of George W. Bush, who was reading a book to Florida schoolchildren when the planes slammed into the World Trade Center. He spent that day being kept out of Washington for security reasons — a decision that then-Sen. Biden urged him to reconsider, the current president has written — and then delivered a brief, halting speech that night from the White House to a terrified nation.

The following year, Bush chose Ellis Island as the location to deliver his first anniversary address, the Statue of Liberty over his shoulder as he vowed, "What our enemies have begun, we will finish."

The wars in Iraq and Afghanistan were still deadly when Obama visited the Pentagon to mark his first Sept. 11 in office in 2009. 

"No words can ease the ache of your hearts," he said. 

"We recall the beauty and meaning of their lives," he said. "No passage of time, no dark skies can dull the meaning of that moment."

By the time Obama spoke at the 10th anniversary, attack mastermind Osama bin Laden was dead, killed in a May 2011 Navy SEAL raid. Though the nation remained entangled overseas, and vigilant against terror threats, the anniversary became more about healing.

President Donald Trump pledged to get the U.S. out of Afghanistan, but his words during his first Sept. 11 anniversary ceremony in 2017 were a vivid warning to terrorists, telling "these savage killers that there is no dark corner beyond our reach, no sanctuary beyond our grasp, and nowhere to hide anywhere on this very large earth."

On Saturday, as Biden was making his way to all three sites, Bush was to pay his respects in Shanksville. Trump planned at least one stop in Manhattan and was to deliver ringside commentary at a boxing match at a casino in Hollywood, Florida.