By Chris Francescani and Ian Simpson
NEW YORK/WASHINGTON (Reuters) - The victims of September 11 will be remembered "no matter how many years pass," President Barack Obama said on Tuesday during one of three main ceremonies marking the 11th anniversary of the attacks in which nearly 3,000 people were killed by airliners hijacked by Islamist militants.
Two of the passenger jets brought down the Twin Towers of New York City's World Trade Center, another hit the Pentagon outside Washington and a fourth crashed in a field in Pennsylvania when passengers aboard that flight fought back against the hijackers.
Obama, speaking at the Pentagon where 184 people were killed, told victims' families that the whole country shares their loss.
"Eleven times we have paused in remembrance and reflection, in unity and in purpose," Obama said. "This is never an easy day, but it is especially difficult for all of you, the families of nearly 3,000 innocents who lost their lives.
"But no matter how many years pass, no matter how many times we come together on this hallowed ground, know this: That you will never be alone, your loved ones will never be forgotten. They will endure in the hearts of our nation because through their sacrifice they helped us make the America we are today, an America that has emerged even stronger."
Speaking under clear blue skies that recalled the crisp morning of September 11, 2001, Obama said America's fight is not with Islam but with al Qaeda, the group responsible for the attacks, and its allies.
It is a line he has used several times since taking office promising to mend ties with the Muslim world.
"I've always said our fight is with al Qaeda and its affiliates, not with Islam or any other religion," he said. "This country was built as a beacon of freedom and tolerance."
Before the Pentagon ceremony, Obama and first lady Michelle Obama observed a moment of silence on the South Lawn of the White House. Afterward they stopped at Arlington National Cemetery, where the two paid their respects at the graves of military service members killed in Iraq and Afghanistan.
Obama's rival for the White House, Republican presidential nominee Mitt Romney, said the anniversary marks a day "when evil descended upon our country."
"On this most somber day, those who would attack us should know that we are united as one in our determination to destroy them and to stand tall for peace and freedom at home and across the world," Romney said in a statement.
'THE PAIN IS SO DEEP'
At Ground Zero in New York where the towers once stood, hundreds of relatives of the victims were joined by local police, firefighters and politicians for the annual reading of the list of the 2,983 people killed at the three sites. The list excludes the 19 hijackers, who died carrying out the attacks.
Cora Fernandez, whose daughter, Judy Hazel Santillan Fernandez, and niece, Maria Theresa Concepcion Santillan, died at the trade center, stood with relatives wearing black T-shirts emblazoned with pictures of the two young women.
"The weather is so beautiful," Fernandez said, "but the pain is so deep. It'll always be there. It'll never go away."
Unlike previous years, politicians were barred from the podium for the name-reading ceremony, which took place alongside the twin reflecting pools that mark the footprints of the fallen towers, their edges etched with the names of the victims. Only families or close friends of the victims participated in the reading, which took more than three hours to complete.
Moments of silence were observed at 8:46 a.m. (1246 GMT), 9:03 a.m., 9:37 a.m. and 10:03 a.m., the times of impact for the four planes, and again at 9:59 a.m. and 10:28 a.m., the times that the north tower and then the south tower fell.
New York Mayor Michael Bloomberg and other politicians, including New York Governor Andrew Cuomo, New Jersey Governor Chris Christie and former New York Mayor Rudolf Giuliani, who was in office when the attacks occurred, gathered near the podium as the names were read.
Tuesday's ceremony occurred just hours after Bloomberg, Cuomo and Christie reached a deal to resume work on a National September 11 Museum at Ground Zero.
A cost dispute between the foundation that controls the museum and the Port Authority of New York and New Jersey, which is building it, has halted work on the underground structure for months.
This year's remembrance also came one day after federal officials agreed that the 70,000 surviving firefighters, police officers and other first responders who raced to the trade center after the attacks will be entitled to free monitoring and treatment for some 50 forms of cancer.
'THEY BARELY KNEW EACH OTHER'
In Shanksville, Pennsylvania, Vice President Joe Biden told families of the 40 passengers who died aboard United Airlines Flight 93 that the nation will always honor their heroism.
They fought back against their hijackers, a move that led to the plane crashing in a remote field 80 miles southwest of Pittsburgh. U.S. authorities say the hijackers planned to crash the plane into the U.S. Capitol in Washington.
Americans have "not forgotten the heroism of your husbands, wives, sons, daughters, mothers, fathers. And that what they did for this country is still etched in the minds of not only you, but millions of Americans, forever," Biden told a crowd of more than 1,000 gathered at the crash site, now a national memorial.
Patrick White, president of Families of Flight 93, said his cousin, Joey Nacke II, and the other passengers were mere strangers only moments before they made their assault on the cockpit where the hijackers had taken control.
"They barely knew each other," White said. "In an astoundingly brief period of time, they came together as one."
(Additional reporting by Drew Singer in Pennsylvania, Matt Spetalnick and Margaret Chadbourn in Washington; editing by Dan Burns and Xavier Briand)