The ceremony in NYC paused six times: twice for when the planes hit the Twin Towers, twice for when each tower fell, once for the attack on the Pentagon, and once more for Flight 93.
The nation is still grappling with the aftermath of 9-11.
The memorial marks the spot where the plane crashed in a rural area at 10:03 a.m.
As New York City marks the 18th anniversary of the terrorist attacks on the World Trade Center on Sept. 11, 2001, the National September 11 Memorial and Museum will be live streaming the 9/11 commemoration ceremony Wednesday morning from Manhattan. The ceremony will include reading the names of all those who were killed. The attacks' aftermath is visible from airport security checkpoints to Afghanistan.
Former President George W. Bush, meanwhile, will attend a wreath-laying ceremony at the Pentagon in the afternoon.
That's where United Flight 93 crashed as passengers and crew attempted to regain control of the plane from hijackers.
"We have to remember that they gave everything for this job, for their communities, for their families", said Lexington firefighter Michael Hurt.
The collapse of the World Trade Center produced thick dust clouds, and fires burned for months in the rubble.
For the 18 anniversary of the September 11, 2001 terror attacks, President Trump is scheduled to attend a ceremony at the Pentagon hosted by Chairman of the Joint Chiefs of Staff Marine General Joseph F. Dunford Jr. and Secretary of Defense Mark Esper.
The sick gained new recognition this year at ground zero, where the new 9/11 Memorial Glade was dedicated this spring.
"We love you, we miss you and you will always be America's heroes", said one woman after reading out the names of her brother and cousin.
President Donald Trump is attending an observance at the Pentagon.
Americans are commemorating 9/11 with mournful ceremonies, volunteering, appeals to "never forget" and rising attention to the terror attacks' extended toll on responders.
Another relative at ground zero underscored that Muslims were among the dead.
Vice President Mike Pence is to speak at the third attack site, near Shanksville, Pennsylvania. A rocket exploded at the U.S. embassy as the anniversary began in Afghanistan, where a post-9/11 invasion has become America's longest war.
There will be moments of silence for when the hijacked planes crashed into the North and South towers of the World Trade Center in NY and the Pentagon in Washington DC and for when United Flight 93 crashed in Pennsylvania after the passengers learned of the other events and attempted to seize control of the plane from the hijackers.