Vice President Joe Biden said the 40 passengers and crew of Flight 93 gave their lives for their country when they overcame four hijackers to crash the plane into a Pennsylvania field.
Flight 93 was a non-stop from Boston to San Francisco and many Bay Area residents were on board.
The Vice President said that while nothing can fill the void left in the hearts of their family members and friends, their actions will always be recalled gratefully, for defining their country and sacrificing their lives so that others could live.
Speaking Saturday at the dedication of the first phase of the Flight 93 National Memorial near Shanksville, Biden said those aboard the plane knew that it was more than a hijacking, but an opening shot in a new war.
He says they acted as citizen patriots have done since Capt. John Parker said in April 1775 that if war is what they want, "then let it begin here."
Former President George W. Bush called the actions aboard Flight 93 on Sept. 11 one of the most courageous acts in U.S. history and a shining example of democracy in action. More than 4,000 people, including relatives of those killed when the plane crashed into a rural Pennsylvania field, attended the service.
He said that even as the U.S. was under attack in New York and Washington, those aboard the hijacked plane defied their four captors by holding a vote to try and overpower them. He said the choice cost them their lives but was successful.
'Determination and valor'
Former President Bill Clinton said he and U.S. House Speaker John Boehner will mount a bipartisan effort to raise the remaining $10 million needed to completely fund the Flight 93 National Memorial, and he praised those aboard the plane as heroes who made the ultimate sacrifice.
He noted that the passengers who overcame the four hijackers were not soldiers, but citizens, who with almost no time acted and gave the U.S. a gift: They saved the capital from attack.
He said they saved lives and denied al-Qaida the symbolic victory of smashing the center of American government.
He said that he hoped that 2,500 years from now, their exploits would not be forgotten.
"They never made it because of the determination and valor of the passengers and crew of Flight 93, that plane crashed in this field, less than 20 minutes by air" from Washington, where it appeared to be headed, said Jon Jarvis, director of the National Park Service.
The Rev. Daniel Coughlin, who was the U.S. House Chaplain at the time of the attacks, called the sacrifices made by the passengers and crew "willing seed for freedom's harvest."
"They refused to be paralyzed. ... They break the silence and decidedly act together. They do only what is possible in an impossible situation," he said in the invocation. "Because they are your children, they find within themselves, true freedoms."
Coughlin's invocation was followed by a long moment of silence as the U.S. flag was brought in, then a singing of "The Star-Spangled Banner." The names of the victims were also read as bells tolled.
Poet Robert Pinsky took to the lectern and read a pair of poems, one about "needing to remember, even if you don't want to," and a second about heroism. The poems came from Brazilian Carlos Drummond de Andrade and Poland's Czeslaw Milosz.
Grammy Award-winning singer and songwriter Sarah McLachlan performed her song of loss and remembrance, "I Will Remember You."