People watch as U.S. President Barack Obama, Defense Secretary Ash Carter and Joint Chiefs Chair Gen. Joseph Dunford take part in a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, U.S., Sept. 11, 2016.
The United States is marking the 15th anniversary of the September 11th terrorist attacks, when al-Qaida terrorists hijacked four planes and flew them into the World Trade Center in New York and the Pentagon near Washington, while one crashed in rural Pennsylvania.
President Barack Obama observed a moment of silence at the White House Sunday at 8:46 a.m., when the first of the four hijacked planes slammed into the World Trade Center, before speaking at a ceremony at the Pentagon, the U.S. military headquarters, honoring those killed in the attacks.
"We remember and we will never forget the nearly 3,000 beautiful lives taken from us so cruelly," Obama said. "We wonder how their lives might have unfolded, how their dreams might have taken shape."
WATCH: President Obama Apeaks at Pentagon Memorial
He vowed that terrorists "will never be able to defeat a nation as great and as strong as America," praising the country's diverse ethnic population comprised of people of all races and religions as "one of our greatest strengths."
Obama, commemorating the September 11, 2001 attacks for the last time as president before leaving office in January, said, "This is the America that was attacked that September morning. This is the America that we must remain true to."
IN PICTURES: 15th Anniversary of 9/11 Attacks
US Marks 15th Anniversary of 9/11 Attacks
1/11Roses are placed on names on the memorial during the ceremony marking the 15th anniversary of the attacks on the World Trade Center at The National September 11 Memorial and Museum in Lower Manhattan in New York City, Sept. 11, 2016.
2/11President Barack Obama places a wreath during a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, Sept. 11, 2016.
3/11People watch as President Barack Obama, Defense Secretary Ash Carter and Joint Chiefs Chair Gen. Joseph Dunford take part in a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, Sept. 11, 2016.
4/11An FDNY firefighter stands as part of an honor guard at the FDNY Memorial Wall near the World Trade Center, Sept. 11, 2016, on the 15th anniversary of the attacks on the World Trade Center in New York.
5/11A mourner holds a photo of her loved one during the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial in New York, Sept. 11, 2016.
6/11A firefighter touches the names of the firefighters, who died in the attacks carved in the south pool, during 15th anniversary ceremony of the attacks of the World Trade Center at the National September 11 Memorial in New York, Sept. 11, 2016.
7/11A woman becomes emotional during a moment of silence in a ceremony for the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial, in New York, Sept. 11, 2016.
8/11Pan Ei Mon, who is married to jailed Reuters reporter Wa Lone, is seen with their daughter after attending a hearing at Myanmar's Supreme Court in Naypyitaw, Myanmar.
9/11A mourner places a flag in the Empty Sky memorial on the morning of the 15th anniversary of the 9/11 attacks in New Jersey, Sept. 11, 2016.
10/11Candles in memory of the passengers and crew of Flight 93, are lit along the Wall of Names at the Flight 93 National Memorial in Shanksville, Pa., Sept. 10, 2016, as the nation marks the 15th anniversary of the Sept. 11 attacks.
11/11The Tribute in Light rises above the lower Manhattan skyline, Sept. 10, 2016, in New York to mark the fifteenth anniversary of the terrorist attacks of Sept. 11, 2001 on the United States.
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Victims killed in the attack on the World Trade Center were remembered in New York, where the country's leading 2016 presidential candidates, Democrat Hillary Clinton and Republican Donald Trump, paid their respects at Ground Zero but halted their political campaigns for the day.
People read the names of the victims during a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial and Museum in New York City, Sept. 11. 2016.
As they have been in past 9/11 commemorations in New York, the names of the 2,983 killed that horrifying day were read slowly by their relatives as music played in the background.
As daylight ends Sunday in New York, spotlights will project two giant beams of light into the sky to represent the fallen twin towers of the World Trade Center.
A woman hugs a man during a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial and Museum in New York City, Sept. 11, 2016.
"Fifteen years ago, a September day that began like any other became one of the darkest in our nation's history," Obama said Saturday in his weekly address.
The president said those killed were "from all walks of life, all races and religions, all colors and creeds, from across America and around the world." It was the worst attack on U.S. soil since Pearl Harbor was bombed in 1941 at the start of World War II.
View the photo gallery of 9/11 attacks:
September 11, 2001 Attacks
1/20The rubble of the World Trade Center smoulders following a terrorist attack.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
2/20The second tower of the World Trade Centre explodes after being hit by a hijacked airplane in New York.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
3/20People walk away from the World Trade Center tower. Three hijacked planes crashed into U.S. landmarks, destroying both of New York's mighty twin towers and plunging the Pentagon in Washington into flames.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
4/20Rescue workers carry fatally injured New York City Fire Depatment Chaplain, Fether Mychal Judge, from one of the World Trade Center towers.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
5/20People hang from the windows of the North Tower of the World Trade Center after a hijacked airliner hit the building.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
6/20A person falls to his death from the World Trade Center after two planes hit the Twin Towers in New York City.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
7/20New York City firefighters and other emergency personnel survey the World Trade Center collapse area near Vessey and Greenwich Streets.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
8/20Firemen carry an injured man from the World Trade Center after both towers collapsed.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
9/20People walk away from the World Trade Center towers in New York City after both towers were hit by airplanes which crashed into the buildings.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
10/20People run from the collapse of World Trade Center Tower in New York.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
11/20A fireball explodes from one of the World Trade Center towers after a jet airliner crashed into the building in New York.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
12/20U.S. President George W. Bush listens as White House Chief of Staff Andrew Card informs him of a second plane hitting the World Trade Center, while Bush was conducting a reading seminar at the Emma E. Booker Elementary School in Sarasota, Florida.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
13/20Fire trucks sit amid the rubble near the base of the destroyed World Trade Center in New York.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
14/20The remaining tower of New York's World Trade Center, Tower 2, dissolves in a cloud of dust and debris about a half hour after the first twin tower collapsed.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
15/20A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airplane crashed into the U.S. military Headquarters outside Washington, D.C.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
16/20A rescue helicopter uses Washington Blvd. outside the Pentagon to evacuate injured personnel after the terror attack on the building.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
17/20Firefighers work on the roof of the Pentagon after a hijacked aircraft crashed into it.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
18/20Thousands of candles are lit in Tallinn, Estonia to commemorate the tens of thousands of victims of the deportation march when, from March 25-28, 1949, more than 22,000 people in Estonia alone were forced from their homes and deported east, many to never return.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
19/20Investigators head into the debris field at the site of a commercial plane crash near Shanksville, Pennsylvania. The crash is one of four planes that were hijacked as part of a deadly and destructive terrorist plot.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
20/20An American flag flies near the base of the destroyed World Trade Center in New York.
As America reflects on the September 11 attacks of 2001, here are some of the iconic images from that tragic day.
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Nineteen hijackers, 15 of them from Saudi Arabia, were killed in the attacks, which led directly to the U.S. war in Afghanistan, where al-Qaida trained attackers against the United States, and indirectly to the war in Iraq. The U.S. still has thousands of forces in Afghanistan and Iraq even as it has ended large-scale combat operations.
Writing on Twitter Sunday, Secretary of State John Kerry said, "On 9/11, we remember those we lost, those who tried to save them. We honor them by pursuing peace, security, justice worldwide."
Near Shanksville in western Pennsylvania Sunday, the Flight 93 National Memorial stands in memory of the passengers and crew members who carried out a sustained assault against the hijackers for control of the plane 15 years ago.
Candles in memory of the passengers and crew of Flight 93, are lit along the Wall of Names at the Flight 93 National Memorial in Shanksville, Pa., Sept. 10, 2016, as the nation marks the 15th anniversary of the Sept. 11 attacks.
A September 11 Museum has been erected on the New York site where the World Trade Center once stood, housing artifacts and photographs connected to the attack.
At the Pentagon, the 184 people who died on September 11, 2001 are honored with 184 benches over pools of water. A huge American flag was draped from the roof of the headquarters of the country's Defense Department on the side of the building where the attack occurred.
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