In a sombre ceremony outside the Pentagon, US President George Bush dedicated the first major September 11 memorial on Thursday, the seventh anniversary of the attacks that killed nearly 3,000 people.
"Seven years ago at this hour a doomed airliner plunged from the sky, split the rock and steel of this building and changed our world forever," Mr Bush said.
"The years that followed have seen justice delivered to evil men in battles fought in distant lands."
To the accompaniment of choral music, military members in dress uniform unveiled the 184 granite-and-steel benches in the memorial park that represent each of the victims killed by the al Qaeda attack on the Pentagon.
The ceremony was the last time that President Bush, who steps down in January, will lead the nation in recalling the attacks that prompted him to declare a global war on terrorism that has defined his presidency.
"Since 9/11, our troops have taken the fight to the terrorists abroad so we do not have to face them here at home," said Mr Bush, joined at the ceremony by former Defence Secretary Donald Rumsfeld, who was in the Pentagon during the attack.
"Thanks to the brave men and women, and all those who work to keep us safe, there has not been another attack on our soil in 2,557 days," he said to applause.
The memorial, by New York designers Julie Beckman and Keith Kaseman, also features maple trees and light pools in a park of gravel. The benches are arranged according to the victims' ages.
The attack on the Pentagon took place at 9:37am, when American Airlines Flight 77 from Washington's Dulles International Airport smashed into the walls of the U.S. military headquarters.
The crash killed 125 people in the Pentagon, along with the plane's 59 passengers and crew and the five hijackers.
In a park near the site of the deadliest Sept. 11 attacks, where more than 2,700 people were killed when New York's World Trade Center was destroyed, relatives of the dead on Thursday held up portraits of their loved ones.
Bagpipes and drums sounded from police and fire department bands.
A ramp with flags of the world led into the pit beneath the site, where a circular reflecting pool contained two squares representing the footprints of each of the twin towers.