But an unusual sight and sound broke the routine at 8:46 that morning. A Boeing 757 passenger plane came in low -- much too low -- over the city's skyscrapers. The thunder of jet engines at full power caused people to stop and look up.
They saw American Airlines Flight 11 from Boston headed for the 417-meter-high north tower of the World Trade Center. At the controls, authorities believe, was Mohammed Atta, the suspected leader of the 11 September hijackers, who had taken control of the plane with four other accomplices. Ninety-four passengers and crew were on board.
Documentary filmmakers happened to be recording on a Manhattan street that day. They caught the roar of Flight 11's engines and their own reactions as it crashed into the north tower, while the sound of the impact reached them moments later.
The plane ripped a jagged, slanting hole across floors 94 through 99 of the 110-story building, engulfing them in an inferno fed by thousands of liters of burning jet fuel. Building fragments, broken glass, and office papers rained onto the streets below.
A few minutes later, live images of the burning tower flashed around the world on CNN.
"This just in. You are looking at obviously a very disturbing live shot there. That is the World Trade Center, and we have unconfirmed reports this morning that a plane has crashed into one of the towers of the World Trade Center," said Carol Lin, a CNN anchor.
The images of what, at first, seemed to be an accident were horrifying. New York police and fire departments began responding to the catastrophe. Ambulances rushed to the scene. All television networks switched to live coverage.
On the U.S. network ABC, a reporter describing the situation reacted in shock as a second hijacked airliner crashed into floors 78 through 84 of the World Trade Center's 415-meter-high south tower. It was 9:03 in the morning -- just 17 minutes after the first plane hit.
"It does not appear that there is any kind of a [rescue] effort up there yet. Now remember.... Oh, my God!"
The second plane was United Airlines Flight 175, also out of Boston, with 53 passengers and crew on board. It is believed that five hijackers took control of that plane and steered it into the World Trade Center.
It was now clear that a coordinated terrorist attack was under way.
In Sarasota, Florida, U.S. President George W. Bush was at an elementary school to talk about a new government reading initiative. He was told of the first plane hitting the south tower, but that it was believed to be an accident. The school visit continued.
A short time later, Bush's chief of staff, Andrew Card, whispered in the president's ear as he sat with a group of children. "Mr. President," Card said, "A second aircraft has hit the World Trade Center. America is under attack."
Before leaving the school, Bush made his first public statement about the attacks.
"I have spoken to the vice president, to the governor of New York, to the director of the FBI, and I have ordered that the full resources of the federal government go to help the victims and their families and to conduct a full-scale investigation to hunt down and to find those folks who committed this act. Terrorism against our nation will not stand," Bush said.
The attacks were not over.
At 9:43 a.m., American Airlines Flight 77, out of Washington's Dulles Airport and carrying 63 passengers and crew, was guided by five suicide hijackers into the Pentagon, the headquarters of the U.S. Department of Defense. About 125 people in the building were killed, along with everyone on the plane.
Back in New York, hundreds of firemen were making their way up the stairs of the World Trade Center towers in an attempt to save lives and fight the raging fires. But the smoke and heat -- which reached some 1,100 degrees Celsius -- proved too much for many trapped people on the upper floors of the buildings, as one witness described.
"There's people jumping out of windows. I've seen at least 13 or 14 people jumping out of windows. It is horrific. I can't believe this is happening."
At 10:05 a.m., the south tower of the World Trade Center collapsed. Intense heat from the fires had melted the building's steel structure, and it caved in on itself.
Thousands of people in lower Manhattan ran for their lives as falling debris and thick clouds of dust filled the streets. Five minutes after the first tower collapsed, a life-and-death struggle was taking place in the skies over the state of Pennsylvania. Four terrorists had taken control of United Airlines Flight 93, heading from Newark, New Jersey, to San Francisco, with 38 passengers and crew, and were turning it toward Washington, D.C.
Passengers aboard Flight 93 had spoken to relatives on their mobile phones and knew of the attacks in New York and at the Pentagon. A small group of passengers is believed to have decided to try to overpower the hijackers and prevent the plane from also being used as a bomb.
The plane crashed a few minutes later in woodland outside the small town of Shanksville, Pennsylvania. Authorities believe the efforts by the passengers may have saved the White House or U.S. Capitol building from being hit.
Back in New York, the intense flames and structural damage proved too much for the World Trade Center's north tower. At 10:28 a.m., it also collapsed in another immense cloud of dust and debris.
More than 2,800 people, including 343 firefighters, were killed in the collapse of the World Trade Center towers.
As the morning progressed, the full scope of the terrorist attacks was still not clear to U.S. national security officials. It was not known if other planes had also been hijacked, or if other sorts of strikes were planned. Air Force One flew the president to a military base in Louisiana, where Bush made his second public statement in the early afternoon. He said the government was taking appropriate steps to protect the American people.
"Our military at home and around the world is on high-alert status, and we have taken the necessary security precautions to continue the functions of your government," Bush said.
As news of the attacks spread throughout the world, foreign leaders rushed to offer the United States their condolences and support. Bush later said Russian President Vladimir Putin had been the first foreign leader to call him after the attacks. Putin also went on Russian television later that day to express the need for solidarity in the fight against terrorism.
"What happened today underlines one more time the importance of the Russian proposal to unite international forces in the fight against terrorism. That is the plague of the 21st century. Russia knows what terrorism is directly, and for that reason, we understand the feelings of the American people," Putin said.
In Berlin, German Chancellor Gerhard Schroeder echoed the sentiments of other European leaders in his reaction to the attack. "This is a declaration of war against the entire civilized world," he said.
In New York, Mayor Rudolph Giuliani was taking charge, inspiring the city and the country with his calm, direct style. After the attack, Giuliani was temporarily trapped when his command post near the World Trade Center was hit by debris from one of the falling towers.
But by 3 p.m., he was holding a news conference and giving the public as much information about the catastrophe as he could. When asked how many people had died, he said, "The number of casualties will be more than any of us can bear." Giuliani also described the massive rescue efforts under way:
"We are trying to evacuate thousands and thousands of people. We have as many of our police and fire personnel as we have down in the southern part of Manhattan evacuating people, trying to save as many lives as possible," Giuliani said.
Later that afternoon, U.S. President Bush was told that his safety still could not be guaranteed in Washington. He was flown to the headquarters of the U.S. Strategic Command at an Air Force base in the state of Nebraska.
The terrorist attack had happened only hours earlier, but speculation was already mounting in the U.S. government and in the media that the attacks were the work of the Al-Qaeda terrorist network. The organization and its leader, Osama bin Laden, were being sheltered in Afghanistan by the Taliban's fundamentalist Islamic government.
At 9 p.m., after returning to Washington, Bush addressed the country from the Oval Office. He said those responsible for the attacks would be found and that the terrorists had only succeeded in rousing a great nation to fight.
More stories on the 9/11 anniversary from RFE/RL:
War On Terrorism Expands To Global Battlefield
Experts Say Winning War on Terrorism Requires Patience, Flexibility
Firefighter's Widow Says, 'We Always Think We Have Tomorrow'
Major Events In Global War On Terrorism In Past Year