WASHINGTON – President Joe Biden is commemorating the 20th anniversary of September 11, 2001, on Saturday with three somber, silent events as he visits the three sites of the worst terrorist attack in modern history.

Biden, along with first lady Jill Biden, is attending ceremonies at the places where four planes and 19 hijackers started America’s 20-year involvement in Afghanistan. His first stop, in New York City, comes amid beefed-up security, city and state officials said, though Mayor Bill de Blasio stressed that there are “no specific and credible threats” against New York.

In prerecorded comments, Biden used the grave occasion to again call for unity in an increasingly divided America.

“Unity is what makes us who we are,” he said. “America at its best – to me, that’s the central lesson of September 11th – is that, at our most vulnerable, in the push and pull of all that makes us human, in the battle for the soul of America, unity is our greatest strength. Unity doesn’t mean we have to believe the same thing. We must have a fundamental respect and faith in each other and in this nation.”

He began his day in New York City, at the site of the World Trade Center, where two planes crashed into the north and south towers on that sunny September morning two decades ago.

President Joe Biden, first lady Jill Biden, former President Barack Obama, former first lady Michelle Obama, and others attend a ceremony marking the 20th anniversary of the Sept. 11, 2001, terrorist attacks in New York, Sept. 11, 2021.

He was joined Saturday morning by former Presidents Bill Clinton and Barack Obama, former Secretary of State Hilary Clinton, and former first lady Michelle Obama, along with senior officials.

Biden will then pay his respects at a field in Shanksville, Pennsylvania, where passengers of Flight 93 revolted against the plane’s hijackers as they attempted to divert the flight to its intended target, the U.S. Capitol.

Vice President Kamala Harris started her commemorations Saturday morning in Shanksville , alongside former President George W. Bush and former first lady Laura Bush.

In the afternoon, Biden will lay a wreath at the Pentagon, the other government target hit by hijackers.

In New York, where the greatest number of people were killed, Mayor de Blasio said this anniversary hits close for many New Yorkers.

“We lost so many people, it’s personal for us,” he said Friday in an event with Governor Kathy Hochul. “And 20 years later, we feel it just as sharply, which is why we are resolved, we say never again. We’re resolved to ensure that terrorists never can perpetrate such an act in this city again, which is why in the months and years after 9/11, this city took it upon ourselves to protect ourselves and build up an extraordinary counterterrorism capacity.”

U.S. Vice President Kamala Harris touches the Flight 93 National Memorial during an event commemorating the 20th anniversary of the September 11, 2001 attacks, in Stoystown, Pennsylvania, Sept. 11, 2021.

Biden, known for his empathy, highlighted that resolve in his remarks Friday.

“On this day, Jill and I hold you close in our hearts and send you our love,” he said. “For people around the world that you’ll never know, who are suffering through their own losses, who see you, your courage – your courage gives them courage that they too can get up and keep going. We hope that 20 years later, the memory of your beloved brings a smile to your lips even while still bringing a tear to your eye.”

But underlying Biden’s messages of love and unity also was a somber reminder of his administration’s uncompromising stance on those who seek to strike the United States. The unity that arose after the event, he said, also showed a steely resolve.

“Unity and resilience, the capacity to recover and repair in the face of trauma, unity in service, the 9/11 generation stepping up to serve and protect in the face of terror to get those terrorists who are responsible, to show everyone seeking to do harm to America that we will hunt you down and we will make you pay,” he said. “That will never stop: today or tomorrow, ever from protecting America.”

People visit the 9/11 Memorial during ceremonies to commemorate the 20th anniversary of the Sept. 11 terrorist attacks, Sept. 11, 2021 in New York City.

The president also drew a subtle numerical line, delivering his sympathies to “the families of the 2,977 people from more than 90 nations killed on September 11th, 2001, in New York City, Arlington, Virginia, and Shanksville, Pennsylvania.”

That was echoed in the New York event, which started at 8:46 a.m. local time with the ringing of a bell to mark the time the first plane hit the North Tower. Family members read names of the victims.

Of the 2,996 people who died on September 11, 2001, 19 were hijackers. By omitting any reference to the 19 hijackers in his comments, Biden communicated – silently – that these killers will have no memorial, no moment, and no forgiveness.