Joe Biden has appealed for a return to the spirit of "true national unity" that took hold in America in the wake of the 9/11 terror attacks on this day 20 years ago.
In a television address on Friday night he said that togetherness was seen in "heroism everywhere", adding: "To me that's the central lesson of 11 September. Unity is our greatest strength."
The US president is to visit the site of all three attacks today, in New York City and Pennsylvania, and at the Pentagon.
Watch President Joe Biden's address to the nation:
His visit comes after his decision to withdraw American forces from Afghanistan last month as the native army the US had spent billions to equip and train collapsed, allowing the Taliban to retake power.
In a six-minute video released on Friday, Biden, 78, reflected on the thousands of lives that were taken two decades ago.
"To the families of the 2,977 people from more than 90 nations killed on September 11, 2001, in New York City, Arlington, Virginia, and Shanksville, Pennsylvania, and the thousands of more that were injured, America and the world commemorate you and your loved ones," he said.
He continued: "We honor all those who risked and gave their lives in the minutes, hours, months, and years afterward.
"The firefighters, the police officers, EMTs and construction workers, and doctors and nurses, faith leaders, service members, veterans, and all of the everyday people who gave their all to rescue, recover, and rebuild. But it's so hard, whether it's the first year or the 20th.
"Children have grown up without parents, and parents have suffered without children. Husbands and wives have had to find ways forward without their partners in their life with them."
The president went on: "No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few days ago.
"So on this day, Jill and I hold you close in our hearts and send you our love."
"We saw national unity bend. We learned that the unity is the one thing that must never break," he noted.
"Unity is what makes us who we are, America at its best. To me, that's the central lesson of September 11th. It's that at our most vulnerable, in the push and pull of all that makes us human, in the battle of the soul of America, unity is our greatest strength."
