Story highlights "We will never forget," House Speaker Boehner says

"Our hearts still ache," President Obama says at the Pentagon

In New York, Washington and Pennsylvania, bells and moments of silence

A bell tolled, ground zero fell silent.

At 8:46 a.m., hundreds who gathered at the site of the fallen World Trade Center towers paused in silence to mark the moment when American Airlines Flight 11 crashed into the north tower -- the opening salvo of a terrorist attack that brought down the iconic buildings, killed 2,977 people and launched more than a decade of war.

Bagpipers broke the silence, and family members of victims of the 1993 bombing of the World Trade Center and the 2001 attack began a solemn reading of the names of those killed at the site.

The 9/11 attack killed 2,753 people in New York, including 403 police and firefighters. The 1993 bombing killed six people.

In Washington, President Barack Obama, Vice President Joe Biden and their wives also paused in silence outside the White House to commemorate the 9/11 victims. The Justice Department also held a moment of silence,

Photos: 9/11 memorial and museum Photos: 9/11 memorial and museum 9/11 memorial and museum – Artifacts from ground zero get a preview at the National September 11 Memorial & Museum in New York. Among them is a cross made out of steel from the World Trade Center in the 2001 attacks. The museum opened in 2014. Hide Caption 1 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – A destroyed New York City Fire Department ambulance from ground zero is on display. Hide Caption 2 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – Cards, patches and mementos of those killed at ground zero -- single objects convey the tragedy of that day, the deadliest terrorist attack on American soil. Hide Caption 3 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – An American flag was recovered from the World Trade Center site. Hide Caption 4 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – Pieces of American Airlines Flight 11 are on display. The plane plowed into the North Tower of the World Trade Center at 8:46 a.m. on September 11, 2001. Hide Caption 5 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – The remains of a New York City Fire Department Ladder Company 3 truck are on display outside the historical exhibition area. Eleven members of Ladder 3 died when the North Tower crumbled. Hide Caption 6 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – Helmets worn by firefighters on September 11, including those of Christian Waugh, were donated by families. Hide Caption 7 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – A photograph at the memorial shows one of the World Trade Center towers collapsing after the attack. Hide Caption 8 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – The symbolic "Last Column," right, a steel beam from one of the towers, stands near the "slurry wall," left, which holds back the Hudson River waters. Hide Caption 9 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – A firefighter shirt from ground zero is on view. Hide Caption 10 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – The wedge-shaped pavilion entrance of the museum, center, is located between the square outlines of the memorial waterfalls at the World Trade Center. Hide Caption 11 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – A visitor to the National September 11 Memorial & Museum takes in the sight. Hide Caption 12 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – A rose is placed next to the name of a victim of the terrorist attacks on the World Trade Center at the North Pool of the memorial. Hide Caption 13 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – Part of the World Trade Center's original foundation, left, and the last column removed from the WTC site, center, are covered in a protective wrap during construction of the museum. Hide Caption 14 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – The "Cross," made of intersecting steel beams found in the rubble of 6 World Trade Center, and a fragment of a trident column, center, one of 84 that formed the exterior structure of each tower, are prepared for display. Hide Caption 15 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – The original stairway from the World Trade Center Plaza to Vesey Street, left, is seen at the museum. Hide Caption 16 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – Contractors work to finish construction of the memorial and museum. Hide Caption 17 of 18 Photos: 9/11 memorial and museum 9/11 memorial and museum – One World Trade Center rises above the lower Manhattan skyline in New York. Hide Caption 18 of 18

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Another moment of silence was held in New York at 9:03 a.m., when the second jetliner, United Airlines Flight 175, crashed into the World Trade Center's south tower.

And at the Pentagon, where American Airlines Flight 77 crashed at 9:37 a.m. on September 11, Obama laid a wreath and then spoke at a private observance for family members of the 184 people who died there.

"Our hearts still ache for the futures snatched away," he said.

In southwestern Pennsylvania, where United Flight 93 crashed into a field near Shanksville, a bell tolled after the name of each of the passengers and crew members was read.

Members of the crew of the USS Somerset rang the bells. The Navy named the ship in honor of Flight 93 passengers who fought back against their hijackers. Forty passengers and crew died when the plane went down.

On the steps of the capitol, members of Congress also gathered to mark the occasion. It was the same place where lawmakers gathered 12 years before in the aftermath of the attacks to demonstrate unity.

"This moment is to pray for the families of the departed, and to ask God to renew our strength and replenish our grace, so that we may press on and serve without growing weary, and walk without growing faint towards that more perfect union of our founders dreams," said House Speaker John Boehner. "That is why we return to these steps today, that is why we will always return. And that is why we will never forget."