Services are being held across the United States and the world to mark 15 years since the 11 September attacks.
Six moments of silence are being held in New York City, to mark the times four hijacked planes crashed and the two World Trade Center towers fell.
Presidential candidates Donald Trump and Hillary Clinton attended a ceremony at the rebuilt site.
Ahead of the anniversary, President Obama said it was important to remember America’s “core values”.
“We’re still the America of heroes who ran into harm’s way; of ordinary folks who took down the hijackers; of families who turned their pain into hope,” he said in his weekly radio address.
“We are still the America that looks out for one another, bound by our shared belief that I am my brother’s keeper, I am my sister’s keeper. In the face of terrorism, how we respond matters.”
Read more: The 9/11 attacks
President Obama held a moment of silence in the White House at 08:46 local time (12:46 GMT), the time the first plane hit the World Trade Center. He will later speak at an event at the Pentagon.
Neither presidential candidate spoke at the New York event, where the names of the victims were read out.
Close to 3,000 people died when planes crashed into the towers, as well as the Pentagon in Washington DC and a field in Shanksville, Pennsylvania.
Fifteen of the 19 attackers were Saudi nationals.
An independent panel completed the 9/11 Commission Report a year after the attacks.
But several sections – informally known as “the 28 Pages” – were withheld from the public for 13 years, fuelling speculation about their contents.
The pages were released in July, and showed it was likely the attackers got financial help from people inside Saudi Arabia, but that there was no official Saudi role.
Last week, the US Congress unanimously passed a bill allowing 9/11 victims’ families to sue the Saudi government.