The mournful tones of "Taps" fluttered over the South Lawn of the White House as President Barack Obama and First Lady Michelle Obama led America in a silent tribute to the victims of the September 11, 2001 terrorist attacks. The crisp, clear day recalled the pleasant fall weather the morning of that national tragedy.

President and Mrs Obama mark the anniversary of the September 11, 2001 attacks.

The Obamas walked somberly out of the residence of the White House and, flanked by hundreds of staff, bowed their heads at 8:46 a.m. — 11 years after American Airlines Flight 11 slammed into the north tower of the World Trade Center. The first couple placed their hands over their hearts as "Taps" played and a military color guard dipped its flags. Afterwards, they turned, clasped hands, and walked back into the presidential mansion.

The Obamas then traveled to the Pentagon for a 9/11 observance ceremony. "This is never an easy day," the president told an audience of active duty service members, families of those killed, and others.

"But it is especially difficult for all of you -- the families of nearly 3,000 innocents who lost their lives -- your mothers and fathers, your husbands and wives, your sons and your daughters. They were taken from us suddenly and far too soon," he said.

"Even now, all these years later, it is easy for those of us who lived through that day to close our eyes and to find ourselves back there -- and back here -- back when grief crashed over us like an awful wave, when Americans everywhere held each other tight, seeking the reassurance that the world we knew wasn't crumbling under our feet," he said.

"And even though we may never be able to fully lift the burden carried by those left behind, we know that somewhere, a son is growing up with his father's eyes, and a daughter has her mother's laugh -- living reminders that those who died are with us still," Obama said.

Relatives of victims of 9-11 listen to President Barack Obama at a Pentagon memorial ceremony.

"As painful as this day is and always will be, it leaves us with a lesson that no single event can ever destroy who we are.  No act of terrorism can ever change what we stand for.  Instead, we recommit ourselves to the values that we believe in, holding firmly, without wavering, to the hope that we confess," he said.  "God bless the memories of those we lost.  And God bless these United States of America."

The president was to visit the Walter Reed National Military Medical Center in the Washington suburb of Bethesda in the afternoon.

Vice President Joe Biden traveled to Somerset County, Pennsylvania, to make remarks at a ceremony honoring the passengers and crew of Flight 93, which slammed into the ground killing all aboard after an insurrection against the hijackers.

In Washington, there were a few signs of some of the changes wrought by the attacks: A K-9 team outside a metro stop a few blocks from the White House, the dark silhouette of a military battery atop an office building overlooking the presidential mansion. And both Obama's campaign and Mitt Romney's suspended negative ads for a day.

The attacks saw extremists from Osama bin Laden's Al-Qaeda terrorist network hijack four airliners to use as guided missiles, crashing into both towers of the World Trade Center, the Pentagon, and a field in Pennsylvania. Some 3,000 people were killed. America's response included the late-2001 invasion of Afghanistan, a war that is now the country's longest, and the March 2003 invasion of Iraq. Osama bin Laden met his end at the hands of Navy SEAL commanders in a May 2011 raid on his compound in Pakistan.