People around the world are remembering the lives of 2,996 people who died on September 11, 2001, 15 years after the attacks.
A commemoration ceremony will take place at the World Trade Center site in New York City at 8:46 am today.
President Obama will make a speech at a Pentagon ceremony, while Vice President Joe Biden will talk at a football game in Philadelphia.
In the years since the attacks, iconic photographs have immortalized the day of devastation, the tragic aftermath and America's rise from the ashes. Here are just some of those pictures:
People look out of the burning North Tower of the World Trade Center.
An American flag flies amid twisted metal.
A lone firefighter walks through the devastation near the base of the towers.
One police officer sits away from grieving family members in the initial aftermath of the attacks.
Part of the Pentagon building destroyed by a commercial jetliner.
A New Yorker is covered from head to toe in dust from the fallen towers.
A man places a flower on a monument engraved with names of victims.
This man was pictured grieving outside the World Trade Center site.
One member of the New York Fire Department places his hand on the memorial.
Ground Zero memorial lights up as people gather to remember the lives of those lost in the attacks.
A view of Lower Manhattan as the sun rises over One World Trade this morning.
Faces of the thousands of lives lost.
Last year, EarthCam released a special time-lapse of the One World Observatory construction which took place over an 11-year period.
The two-minute clip posted on YouTube shows the building progress from October 2004 to Memorial Day 2015.
It was created by fusing hundreds of thousands of high-definition images captured by EarthCam's cameras.