AZUZ: Observances were also held around the world to remember those lost, including hundreds from other countries. And in New York, a memorial opened as a permanent tribute to the events of 9/11.
The memorial took several years to plan, and it had many revisions along the way. And nearby a tower even taller than the fallen towers is under construction.
To help us all remember, Anderson Cooper looks back at the World Trade Center`s history, its significance and the new tower that`s rising.
ANDERSON COOPER, CNN: (voice-over): New Yorkers have been talking about building a world trade center for 20 years before ground was broken on Manhattan`s Lower West Side on August 5th, 1966. Older buildings had to be demolished.
The North Tower started going up in 1968, the South Tower five months later. The first tenants moved in in 1970, even before construction finished on the upper floors. Ribbon cutting was in 1973.
The towers were full of innovations. At 110 stories, they were the tallest buildings in the world, at least for a little while. Each floor was about an acre of open space, their weight distributed between a central core and steel columns in the building`s outer skin.
High-speed express elevators and sky lobbies on the 44th and 78th floors made getting to the top quick and efficient. The complex even had its own zip code, 10048. Iconic additions to Manhattan`s skyline, the World Trade Center never stopped attracting attention. But as the years went by, the towers, symbols of a city, a country and a way of life, also became a focal point for hatred.
In February 1993, a van packed with explosive was detonated in a package garage under the North Tower. Six people died, and about a thousand were hurt. Islamic extremists behind the attack were rounded up, tried as criminals, convicted and sent to prison. But the international terrorists who inspired them kept plotting, and struck on that crystal clear morning in 2001.
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UNIDENTIFIED FEMALE: A plane has crashed into one of the towers of the World Trade Center.
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COOPER (voice-over): It took eight months for bodies to be recovered and for a million tons of twisted steel and concrete to be cleared away.
Plans for a new and even taller skyscraper were revealed quickly, and changed repeatedly to make it stronger and safer. The new One World Trade Center will have a reinforced center core, extra fireproofing, biochemical filters and even green technology.
Groundbreaking for the main tower, One World Trade Center, took place in 2006.
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GEORGE PATAKI: We are going to soar to new heights and reclaim New York`s skyline with this magnificent symbol of our freedom.
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COOPER (voice-over): Today, the still unfinished tower just pokes above the skyline on its way to becoming the country`s tallest skyscraper, 1,776 feet at the tip of its antenna, matching the year of U.S. independence, 1776.
AZUZ: In Washington, D.C., mourners gathered at the Pentagon, which also came under attack on 9/11.
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AZUZ (voice-over): Here, there was a moment of silence at 9:37 am, when in 2001, a plane struck the Pentagon, killing 184 people.
Vice President Joe Biden addressed those gathered, and spoke of America`s resolve in the midst of a historic challenge.
JOE BIDEN, VICE PRESIDENT OF THE UNITED STATES: Al Qaida and Bin Laden never imagined that the 3,000 people who lost their lives that day would inspire 3 million to put on the uniform and harden the resolve of 300 million Americans.
They never imagined the sleeping giant they were about to awaken. They never imagined these things, because they did not understand what enables us, what has always enabled us to withstand any test that comes our way. But you understood.
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AZUZ: A third ceremony was held in a field in Shanksville, Pennsylvania. That`s where another plane went down after passengers realized the hijackers` plans and tried to stop them from hitting a fourth target, thought to be the Capitol building in Washington.
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AZUZ (voice-over): Sunday, President Obama and the first lady laid a wreath in honor of the 40 passengers and crew members who lost their lives in that crash. It came a day after a new memorial was unveiled at the site.
David Mattingly has more on how the men and women aboard that flight will be remembered.
