Sept. 7, 2011 -- The terrorist attacks at the World Trade Center in New York City and the Pentagon building in Washington, D.C. on September 11, 2001 changed life in the United States forever. Before that day, a terrorist attack on American soil was unthinkable.
Reactions included shock, terror, sadness -- all captured in the faces of those that watched the horror unfold that day.
Amid the chaos, there were unsung heroes helping those injured. The photo above shows two unidentified men, one helping the other, outside the Pentagon.
IMAGES: Afghan War Pictures
This photo shows workers and police officers in lower Manhattan.
A group of women help each other toward safety.
Two onlookers, covered in dust from the destruction at the World Trade Center.
Twenty-eight-year-old Macy Borders sought shelter in a nearby building after the collapse of the World Trade Center. She had been walking nearby when she was enveloped by the cloud of dust and smoke.
Stan Honda, a photographer, was seeking shelter in the same building and captured the moment. "She was sort of this ghostly figure," Honda told LIFE.com, "covered in grey-white dust, and I thought that this was an amazing thing to see, that this would make an important picture of what was happening out there."
As the news spread, so did the shock. In this photo, students at Bringham Young University in Utah react to a television report.
More from LIFE
See more images from LIFE.com
9/11: The 25 Most Powerful Photos
In Praise of 9/11 First Responders
In They Were There: 9/11 Photographers