President Obama marked the 12th anniversary of the Sept. 11 terrorist attacks Wednesday, calling for Americans to honor the victims and their families on a day of national remembrance.

“Together, we pause, and we pray and we give humble thanks as families and as a nation for the strength and the grace that from the depths of our despair has brought us up again,” the president said from the Pentagon. “Our hearts still ache for the futures snatched away, the lives that might have been.”

Obama, first lady Michelle Obama, Vice President Joe Biden and Jill Biden observed a moment of silence from the South Lawn of the White House Wednesday morning.

A bell tolled three times at 8:46 a.m., marking the moment 12 years ago when American Airlines Flight 11 crashed into the north tower of the World Trade Center.

The first family and the Bidens placed their hands over their hearts as a military band sounded Taps.

The president later participated in a wreath-laying ceremony at the Pentagon to honor the victims killed when another hijacked plane was flown into the building on 9/11.

Obama stood alone in front of the wreath at the base of the Pentagon Memorial, with a massive American flag draped from the top of the building.

“Above all, let us have the courage — like the survivors and families here today — to carry on no matter how dark the night or difficult the day,” Obama said.

Over on Capitol Hill, members of Congress were also paying their respects to the families and victims of the Sept. 11 attacks.

“Yes, we have been through the crucible, and we live in a dangerous world. But from the fallen – and from all who have sacrificed so that we may live free – we can take heart that ours is the greatest cause and the work before us is not above our capacity or beyond our strength,” House Speaker John Boehner, R-Ohio, said from the Capitol steps. “After all, we are Americans.”

Obama later Wednesday will participate in a service project in the Washington, D.C. area.

The White House also sought to assure Americans they were on high alert a year after the deadly Sept. 11 terrorist attack on the U.S. Consulate in Benghazi, Libya, which took the lives of four Americans, including Ambassador Christopher Stevens.

The White House said that counterterrorism officials for months have been reviewing security procedures in preparation for the anniversary of Sept. 11, guarding against possible threats to American facilities overseas.

"We remain committed to bringing the perpetrators of the Benghazi attacks to justice," White House Press Secretary Jay Carney said late Tuesday, "and to ensuring the safety of our brave personnel serving overseas."