The cloudless sky was little portent of the dark, history-changing day that was about to unfold.
As New Yorkers began heading to work, 19 hijackers were boarding flights at airports in Boston, Washington and Newark.
The Islamist extremists, overwhelmingly from Saudi Arabia, were carrying knives, which at the time were allowed on planes if the blade was less than four inches (10 centimeters) long.
At 7:59 am, American Airlines Flight 11 departed Boston's Logan Airport, bound for Los Angeles. On board were 92 people -- five of them were hijackers, including ringleader Mohamed Atta.
Sixteen minutes later, United Airlines Flight 175 left the same airport, also bound for LA. Sixty passengers and crew were on board, along with five hijackers.
Around the same time, on Flight 11, a hijacker stabbed a passenger, who became the first victim of 9/11. The jihadists wrested control of the plane and turned it towards New York.
A few minutes later, outside the US capital, American Airlines Flight 77 took off from Dulles airport, bound for LA. Six crew, 53 passengers and five hijackers were on board.
And at 8:42 am, in Newark, New Jersey, United Airlines Flight 93 departed for San Francisco.
Those four planes would not reach their intended destinations.
'I was so afraid'
In downtown Manhattan, the 50,000 people who worked at the World Trade Center, symbol of America's economic might and site of New York's two tallest skyscrapers, began to stream into their offices.
Among them was Joseph Dittmar, one of several witnesses interviewed by AFP in the run-up to the 20th anniversary of the attacks.
That day, Dittmar -- who was then a 44-year-old Chicago-based insurance expert -- took the elevator to the 105th floor of the 110-story South Tower for an 8:30 am meeting.
At 8:46 am, the lights in the windowless room flickered. The 54 meeting participants were told to evacuate. They didn't know it at the time, but Flight 11 had just smashed into the neighboring North Tower.
Dittmar and his colleagues shuffled down to the 90th floor, where they caught their first glimpse of the horror unfolding.
"That was the worst 30-40 seconds of my life," Dittmar, now 64, told AFP.
"To see these huge black holes through the sides of the building, flames redder than any red I had seen before in my life, gray and black billows of smoke just pouring out of these big holes.
"We saw furniture, paper, people being pulled out of the building against their will. I was so afraid," he told AFP, choking back tears.
'America is under attack'
As many around him became transfixed by what they were witnessing, Dittmar headed for the stairs to resume his descent -- a decision that would save his life.
On the ground, chef Michael Lomonaco emerged from the shopping center beneath the World Trade Center, where he had been delayed by a last-minute decision to get his glasses fixed.
He looked up at his Windows on the World restaurant on the North Tower's 107th floor.
"I could see people waving tablecloths from the windows of our restaurant. It was horrible, terrible," he recalled.
Word about the crash quickly spread across the Big Apple, where voters were casting ballots to elect a successor to mayor Rudy Giuliani.
Residents struggled to comprehend how a pilot would fail to see a giant skyscraper. On television news networks, anchors speculated about what had caused the "accident."
At 8:50 am, President George W. Bush, visiting an elementary school in Florida, was informed that a small plane had tragically, but accidentally, hit the North Tower.
Meanwhile, air traffic control in New York was trying to make contact with Flight 175.
There was no response; minutes earlier, hijackers had taken control of the Boeing 767 above New Jersey.
As emergency services rushed to evacuate people from the North Tower, an announcement came over the loudspeaker in the South Tower, telling occupants to stay put. The building is secure, it said.
Dittmar ignored colleagues who told him to take one of the express elevators from the 78th floor to the ground. He knew to stick to the stairs during fires.
Somewhere between the 74th and 75th floor, the stairwell started to "shake so violently back and forth," Dittmar recalled.
"The handrails break away from the walls, the steps underneath our feet undulate like waves in the ocean. We feel this heat wall, smell this jet fuel," he said.
It was 9:03 am. Dittmar was unaware that jihadists had flown Flight 175 into the South Tower, striking just above him between floors 77 and 85. The crash is broadcast live around the world.
At Emma Booker Elementary School in Sarasota, Bush was reading "The Pet Goat" to second graders.
His chief of staff Andy Card interrupted him to say a second plane had hit the World Trade Center. He whispered in the president's ear: "America is under attack."
At 9:30 am, Bush made a brief statement, calling the blasts "an apparent terrorist attack." He ordered "a full-scale investigation to hunt down" the perpetrators.
- 'The looks in their eyes...' -
Back at the South Tower, some office workers started to do what others had already been doing in the North Tower: trapped on high floors and with no escape, they jump to their deaths.
On floor 31, Dittmar passed firemen and other first responders coming up the stairs.
"The looks in their eyes (said) they knew they were going up and they were never coming back," he said.
When Dittmar and his colleagues emerged on the ground floor, they found large chunks of steel and cement strewn across the ground -- and huge blood stains.
As debris rained down from above, rescuers directed them towards the shopping concourse under the World Trade Center. They exited a few blocks north.
At 9:59 am, they heard the deafening sound of the South Tower collapsing behind them.
Almost instantly, "the cry of tens of thousands of distraught" witnesses rang around Lower Manhattan, which was quickly blanketed in a cloud of toxic ash.
'This is the safest place'
Across the United States and around the globe, millions of viewers glued to their televisions were dumbfounded by what they saw.
Amid the rubble, 37-year-old paramedic Al Kim's entire body was covered with thick ash. Heat had burned his nostrils, upper respiratory tract and eyebrows.
He had rushed to the World Trade Center to help evacuate the wounded to the nearby Marriott Hotel just before the South Tower fell.
"I couldn't breathe. The air was so acrid. I remember using my shirt to cover my mouth. I couldn't even see my hands in front of my eyes," he recalled.
Ringing around him were alarms carried by firefighters, which go off when they are motionless for a while.
Kim detected the voices of two colleagues. They met and held hands "like schoolchildren," inching their way through rubble, darkness and fire.
Thousands of feet in the air, further drama was unfolding.
At 9:25 am, the Federal Aviation Administration had stopped all flights from taking off. Fifteen minutes later, it ordered all civilian planes in US airspace to land.
The directives are unprecedented in the history of American aviation.
But they come too late to stop hijackers from taking control, shortly before 9:00 am, of American Airlines Flight 77, which headed towards the US capital.
Inside the Pentagon, just outside Washington, media relations specialist Karen Baker didn't think she was in danger.
"This is the safest place to be in the world right now," the then 33-year-old remembered telling a co-worker.
But at 9:37 am, as she returned to her desk from the cafeteria, her illusion was shattered as hijackers slammed Flight 77 into the headquarters of the US Department of Defense.
Everyone aboard perished, along with 125 people in the building.
"It was a loud boom and then you felt a shaking. At the time we were thinking it was a bomb that had been let off somewhere in the building," Baker recalled.
At roughly the same time, the White House was evacuated and Vice President Dick Cheney was hurried into an underground bunker.
Revolt in the sky
Shortly before 9:30 am, in the skies above Ohio, United Airlines Flight 93 was hijacked by four armed men.
As the plane descended, passenger Edward Felt, a 41-year-old father-of-two, called the emergency hotline 911 from the bathroom to report the hijacking.
During calls with relatives, some of the 33 passengers and seven crew learned about the two jetliners that struck the World Trade Center.
A group of them decided to storm the cockpit to prevent the hijackers from crashing Flight 93 into their intended target, later believed to be in Washington.
One of them, Todd Beamer, was heard saying, "Are you ready? Okay, let's roll," during a call with a phone operator.
The battle lasted six minutes. As the passengers tried to force open the cockpit door, the hijackers rocked the plane and tried to cut off its oxygen.
At 10:03 am, Flight 93 crashed into a wooded area in the small Pennsylvania town of Shanksville, 75 miles (120 kilometers) south of Pittsburgh.
The plane, loaded with fuel, exploded in a huge fireball that instantly killed everyone on board. No one on the ground was killed.
Felt's brother, Gordon Felt, received a phone call from his sister-in-law at the summer camp he is running in upstate New York. She told him Edward was on Flight 93.
He left a message on his brother's cell phone: "Ed, when you land, call us. We're worried."
Air Force One flies high
Meanwhile, Air Force One, unsure where to take the president, ascended above the Gulf of Mexico to the unusually high altitude of 45,000 feet, where communications are virtually non-existent.
Bush wanted to go back to Washington but the Secret Service, worried that there might be more hijacked planes heading for the capital, deemed it too dangerous. They took him west -- to Barksdale Air Force Base in Louisiana.
Back in New York, at 10:28 am, the North Tower collapsed after burning for 102 minutes. Giuliani urged residents to remain "calm" -- but told them to evacuate Lower Manhattan.
With the subway system shut, hundreds of thousands set off on foot. Many walked for hours, north to Upper Manhattan or east across the Brooklyn Bridge to Brooklyn.
An impromptu flotilla of ferries, fishing boats, yachts and coast guard vessels evacuated others by water. They carried an estimated 500,000 people throughout the day in a maritime evacuation larger than Dunkirk.
Pledge to 'defend freedom'
Frantic rescue efforts amid the World Trade Center's smoky ruins of what journalists were calling "Ground Zero" continued throughout the afternoon.
Many rescuers, volunteers and trained personnel later developed lung cancer from breathing in the poisonous dust.
Paramedic Kim helped free firefighter Kevin Shea, who would be the only survivor from his brigade of 12.
At 12:30 pm, 14 people emerged alive from a staircase that remained intact in the North Tower. Their escape became known as the "Miracle of Stairway B."
Chef Lomonaco tried to establish which employees were at the restaurant but many were unreachable.
It would take days to learn that there were 72 on site -- none of them survived.
From Louisiana, Bush announced that the US military was on "high alert." He was then flown to a base in Nebraska.
Edward Felt learned there were no survivors on Flight 93 and called his mother to tell her that Gordon was dead.
Dittmar took a train to his parents' home in Philadelphia. He remembered there was "total silence" in the train car -- everyone was in shock.
He collapsed with fatigue and missed Bush's 8:30 pm address to the nation from the Oval Office in which the president announced that thousands had died.
Bush warned that Washington would not distinguish between "the terrorists who committed these acts and those who harbor them."
"None of us will ever forget this day, yet we go forward to defend freedom and all that is good and just in our world," he said.
Back home, the enormity of what Baker had survived slowly sank in as she hugged her husband and two young boys.
"The sheer tension had pushed them to the edge and they were just sobbing," she recalled.
Kim didn't get home to Brooklyn until midnight. He showered, rested for a few hours and woke up early.
"There was a lot to do, there were funerals to go to. There was no moment to reflect," he remembered.
