US President Joe Biden has urged unity as his country remembers the victims of the 2001 11 September attacks.
In a video released on the eve of the 20th anniversary, he paid tribute to the 2,977 people who lost their lives.
"We honour all those who risked and gave their lives in the minutes, hours, months and years afterwards," Mr Biden added, speaking of the emergency workers who responded to the attacks.
Commemoration events are due to take place on Saturday.
Joe Biden will begin with a ceremony at 08:30 (12:30 GMT) in New York at the site where the World Trade Center's Twin Towers once stood.
He is not expected to deliver any speeches, but in his address on Friday, he said: "No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago."
He acknowledged the "darker forces of human nature - fear and anger, resentment and violence against Muslim Americans" which followed the attacks, but said that unity had remained the US' "greatest strength".
"We learned that unity is the one thing that must never break," he added.