NEW YORK (AP) — Three American presidents stood somberly side by side on Saturday at the National September 11 Memorial in New York, sharing a moment of silence to mark the anniversary of the nation’s worst terrorist attack with a display of unity.

Presidents Joe Biden, Barack Obama and Bill Clinton gathered at the site where the World Trade Center towers fell two decades ago. Each man wore blue ribbons and held his hands over his heart as a procession marched a flag through the memorial before hundreds of people, 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. For much of the ceremony he stood with his arms crossed and head bowed, listening while the names of the victims were read. At one point, he wiped a tear from his eye.

Biden was a senator when hijackers commandeered four planes and carried out the attack. He was Obama’s vice president in 2011 when the United States observed the 10th anniversary of the strikes. Saturday’s commemoration was his first as commander in chief, culminating with a stop late afternoon at the Pentagon, where the world’s mightiest military suffered an unthinkable blow to its very home.

In between he visited Shanksville, Pennsylvania, where passengers brought down a hijacked plane that was headed for the U.S. Capitol. Biden and his wife, Jill, walked with relatives of the crash victims into the grassy field when the jet came to rest.

On a day when his nation recalled its shock and sorrow, Biden left the speech-making to others.

It is now Biden who shoulders the responsibility borne by his predecessors to prevent another strike. He must do that against fears of a rise in terrorism after the hasty U.S. withdrawal from Afghanistan, the country from which the Sept. 11 attacks were plotted.

Biden’s vice president, Kamala Harris spoke at the Flight 93 National Memorial, praising the courage of those passengers and the resilience of Americans who came together in the days after the attacks.

“In a time of outright terror, we turned toward each other,” she said. “If we do the hard work of working together as Americans, if we remain united in purpose, we will be prepared for whatever comes next.”

Former President George W. Bush, speaking before Harris, recalled how 9/11 showed that Americans could unite despite their differences. It was a message, he said, that was needed today.

“So much of our politics have become a naked appeal to anger, fear and resentment,” Bush said. “On America’s day of trial and grief, I saw millions of people instinctively grab for a neighbor’s hand, and rally for the cause of one another. That is the America I know.”

That was a theme that Biden emphasized in a taped address released by the White House late Friday. He spoke about 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’s task, like recent presidents, was to mark the moment with a mix of grief and resolve. A man who has suffered deep 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: “No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago.”

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.

Bush 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 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 terrorist attack would define Bush’s presidency. The following year, he chose Ellis Island as the location to deliver his first anniversary address, the Statue of Liberty over his shoulder as he pledged, “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.

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 terrorist 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.”

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Jaffe reported from Washington.