Americans have gathered to commemorate the anniversary of 9/11, 22 years after the deadliest terror attacks on US soil.
Relatives read aloud the names of nearly 3,000 people who were killed in the al-Qaeda attacks.
"It's 22 years and this is the way I still feel, like it was yesterday," Sybil Ramsaran, who lost her daughter, said at a memorial event in New York.
President Joe Biden chose to mark the day in Alaska.
"I join you on this solemn day to renew our sacred vow - never forget," he said to US troops at Joint Base Elmendorf-Richardson in Anchorage.
Shortly before the anniversary, officials in New York City identified two more victims, according to the mayor's office.
The man and woman are the 1,648th and 1,649th victims to be identified by the Office of the Chief Medical Examiner. Their names have been withheld at the request of their families.
Commemorations were held on Monday at the sites of the attack - at New York's World Trade Center, the Pentagon in Virginia, and Shanksville, Pennsylvania, where a plane crashed into a field after passengers overpowered the hijackers.
Vice-President Kamala Harris joined victims' families at the 9/11 memorial in New York, where the twin towers of the World Trade Center fell.
"I wish I had a chance to really know you. Everyone in the family misses you. We will never forget," said the grandson of firefighter Allan Tarasiewicz, who died aged 45 during the rescue there.
At the Pentagon, bagpipes and The Star-Spangled Banner were performed in front of military personnel and civilians. People could be seen saluting and holding their hands over their hearts.
"September 11 made America a nation at war, and hundreds of thousands stepped up to serve our country in uniform," Defense Secretary Lloyd Austin said at the Pentagon ceremony.
"I know that it aches to remember this milestone year after year... The men and women of the Department of Defense will always remember."
Moments of silence, tolling bells, candlelight vigils and other activities were held across the country on the day the US Department of Defense and others now call Patriot Day.
Boy Scouts and Girl Scouts raised and lowered the US flag at a commemoration in Fenton, Missouri, the location of a Heroes Memorial that includes a piece of steel from the World Trade Center and a plaque honouring a 9/11 victim.
"We're just a little bitty community," said Fenton Mayor Joe Maurath, but "it's important for us to continue to remember these events. Not just 9/11, but all of the events that make us free", the Associated Press reported.
In New Jersey's Monmouth County, 9/11 was made a holiday this year for county employees.
Members of Minnesota's St Joseph Fire Department spent time on Sunday doing the annual 9/11 stair climb, climbing to the top of the hose tower and back down 28 times, according to local news.
The 2,240-stair march simulated the 110 stories climbed by firefighters in the World Trade Center 22 years ago, and honoured the lives of more than 300 firefighters who died during rescue efforts.
The 9/11 attacks were the deadliest assault on US soil since the 1941 Japanese attack on Pearl Harbour, Hawaii, where 2,400 people were killed.