It was the kind of public event he enjoyed the most. Nowhere was the president more comfortable than sitting in front of a class of seven-year-olds, listening to the schoolchildren read a book out loud. Behind him, pinned to a blackboard, was a simple handwritten sign: "Reading makes a country great!"

President George W. Bush appeared relaxed as he took his seat inside the Emma E. Booker elementary school in Sarasota, Florida. He had already heard the outlines of what his advisers were calling "a crash" in New York. But if the 43rd US president was concerned by the phone call with his national security adviser, he was not showing it.

Soon after the 16 children began their reading exercise, Andrew Card, White House chief of staff, quietly stepped up to the head of the class to whisper in the president's ear.

It was not just a crash. A second hijacked airliner had torn into the second tower of the World Trade Center in lower Manhattan.

The president's jaw tensed, and he nodded. But he maintained his composure long enough to question the children about a phrase in their book - "more to come".

"What does that mean?" he asked, "more to come."

Twenty minutes later, a grim-faced president was standing in the school's media room before a 200-strong audience of local officals, teachers and media. "Ladies and gentlemen, this is a difficult moment for America," he began. "I unfortunately will be going back to Washington after my remarks.

"Today, we've had a national tragedy. Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country. Terrorism against our nation will not stand."

By the time the president's motorcade reached Air Force One at Sarasota airport, the president heard that the Pentagon - Washington's most fortified building - had been struck by a third airliner.

As Mr Bush scrambled on board his 747 jumbo jet, emergency alarms were sounding across Washington. The White House, Treasury and Capitol were evacuated and the nation's leaders were spirited away to secret bunkers.

In the skies above Florida, even the White House staffers on board Air Force One had no idea where they were headed. Secret Service agents on board - the president's bodyguards - could only watch the local TV news beamed into the press cabin as the twin towers of the World Trade Center crumbled into a cloud of debris. Some 200 of their colleagues in the New York field office were inside the office complex.

Within an hour, an anonymous call reached the Secret Service. Using code words known only to the agency's staff, the caller issued a chilling warning: "Air Force One is next."

The presidential plane - known as the flying Oval Office - climbed steeply to 40,000ft. "We want to get the plane up and we want to get it up very high," the president was told by the head of his Secret Service detail.

"I don't want some tinhorn terrorist keeping the president of the United States away from the nation's capital," Mr Bush insisted.

But the president was cautioned by his security detail against heading home. Another six aircraft were reported to be unaccounted for. Any of them could be heading towards the White House and the president himself.

Fighter jets appeared at the wings of the president's plane as it began its descent towards Barksdale Air Force Base in Louisiana. On the ground, the president was surrounded by guards in full combat gear armed with M-16 rifles. His motorcade, complete with a Humvee vehicle carrying a gun turret, took him to headquarters building 245. There the windows carried a stark black and white sign: Def Con Delta. The US military was on its highest state of alert.

Ninety minutes later, the president climbed into the camouflaged Humvee and boarded Air Force One once more. Instead of heading east to the White House, he flew west again to Offutt Air Force Base in Nebraska.

It would be almost 10 hours after leaving the earnest schoolchildren in Sarasota before President Bush would return to the capital of a nation which had changed utterly.

It was still dark on Tuesday morning when two men of Middle Eastern origin pulled their silver Nissan Altima rental car into the Jetport in the small coastal city of Portland, Maine. Casually dressed, they lit cigarettes for a last smoke in the parking lot before entering the terminal.

Inside, they checked their luggage; one of the bags - bearing a tag with the passenger's name in Arabic - held a how-to-fly manual, a fuel-consumption calculator and a copy of the Koran. Despite a clear sky, the 5:45 American Airlines flight from Portland was slightly delayed. Not until 6:00am did the men board the plane for the one-hour trip to Boston.

About 100 miles away, in Dracut, Massachusetts, 50-year-old John Oganowski was leaving his farm to report for duty as a pilot at American Airlines on Flight 11 en route to Los Angeles. Mr Oganowski was looking forward to a family picnic at the weekend. As he drove past his uncle's house nearby, he gave a customary toot-toot on his car horn. The sun was coming up over the horizon. On this unusually warm and clear late summer morning, Mr Oganowski did not expect any flight delays.

Other passengers were also getting ready for the six-hour flight to Los Angeles. Danny Lewin, co-founder of the Internet company Akamai Technologies, left his home in Brookline for Flight 11. Two friends, Ruth Clifford McCourt, 45, and Paige Farley Hackel, 46, looked forward to vacationing together in California. They had tried but failed to get reservations on the same flight. Ms McCourt was headed for United Flight 175 while Ms Hackel went for American Flight 11.

At Logan Airport, a group of Arab men parked their rented white Mitsubishi sedan. The car had already been seen by video cameras at the airport at least four times that week. In the parking lot, the men were seen in a heated argument, an incident later reported to police. Inside the car was a instructional flight video and a "ramp pass" that allows the carrier access to restricted areas at Logan Airport.

Captain Ognowski and his first officer Thomas McGuinness were running over the checklist in the cockpit as the 81 passengers boarded Flight 11 to Los Angeles.

Waiting behind for his turn to be pushed back from the gate Capt Victor Saracini was at the controls of United Airlines Flight 175, also bound for LA.

On Flight 11 the cabin attendants checking seatbelts and the overhead luggage bins could note some of the better known passengers; Berry Berenson, fashion photographer and accomplished film actor, who was married for 20 years to Anthony Perkins before his death from Aids in 1992; David Angell, sitting beside his wife Lynn, one of the best comedy writers in Hollywood, he had worked on the sitcom Cheers about a Boston pub and had helped to create the popular series Frasier.

For five of the passengers, the empty aircraft - a sign of the recent economic downturn - helped dispel any last minute doubts about whether they could succeed in what they were about to do. The fewer people on board the less chance anything could go wrong now.

Mohammed Atta, who carried a United Arab Emirates passport and had an Egyptian driver's licence, was seated in business class seat 8D as the aircraft left the ground exactly on time at 7:59am. For five months from June last year, Atta, now aged 33, had attended Huffman Aviation flying school in Florida and then honed his sklls on a commercial jet simulator at Opa-Locka airport near Miami. Now he no longer needed the Arabic-language Boeing flying manual - nor the Koran - left behind in the parked car at Boston airport.

Just minutes later screams filled the cabin as five men began to attack the nearest flight attendants with knives. At the cockpit door they left the pilots in no doubt that their colleagues would die unless they were let on to the flight deck.

"Don't do anything foolish, you won't be hurt," the hijackers assured Captain Ognowski. The chilling words were picked up by startled air traffic controllers. Their alarm grew at the disappearance of Flight 11's call sign on their radar screens, as the hijackers turned off the automatic transponder signal.

Flight 175 and Flight 11 began to veer from their scheduled routes. As they headed for New York, they nearly collided in mid-air. Travellers began to make furtive cell phone calls to family members. A flight attendant called to identify one of the hijacker's seat numbers, information that would later help police. Peter Hanson of Groton, Massachusetts, on board United Flight 175, managed to get through to his father. "Something's wrong with the plane," he said. "Oh my God! They just stabbed the airline hostess."

Minutes later air traffic controllers realised that Flight 77 from Dulles Airport just outside of Washington, on route to LA, and Flight 93 from Newark to San Francisco were also in the hands of hijackers.

About an hour later Robert and Andrea Souza were sitting in first class on an American Airlines flight to Newark from Florida scheduled to land around 10am. The newly-weds had enjoyed a long weekend away from the daily stress and grind of New York.

Mrs Souza, a frequent traveller, was suddenly struck with a feeling of terror as she observed the movements of the stewardess in front of her. "Something is wrong," she said, grabbing her husband's arm.

The flight attendent's eyes had gone wide, then began filling with tears as she exchanged whispers with another steward. She began to make an announcement, "Ladies and gentlemen..." then broke down. "I can't do this," she mouthed to another attendant.

At 6:40am Elliott Nadel left his home in the Bayside area of Queens and got behind the wheel of his new Mercedes-Benz.

Setting out for his office 18 miles away in the Northern Tower of the World Trade Center, where he worked as an investment manager, he glanced with approval at the cloudless sky.

After crossing over to Manhattan through the Lincoln Tunnel he drove south along the FDR Drive. At around 7.30 he parked the car, which had been delivered less than three weeks earlier, in an underground garage and walked the remaining few blocks to the World Trade Center.

As he approached the rotating doors at the base of the 110-storey glass and steel North Tower his mind was on the market. Stock prices had been falling for the past few weeks, and the married father of two was hoping that today would mark the beginnings of a rally.

Hundreds of other workers crossed the plaza between the 1,250ft towers, on their way to work at the scores of banks, law firms and services companies that had offices in the complex. In the nearby Marriott hotel, delegates of the National Association for Business Economics were preparing for a speech by Robert Scott, the president and chief operating officer of Morgan Stanley. High above, in the Windows of the World restaurant on the 106th floor, the power brokers of New York's financial district were enjoying breakfast accompanied by spectacular views of the city.

The lift zipped Mr Nadel up to the 78th floor where he got out and crossed over to the other elevator bank. As he had done countless days before, he got out on the 85th floor and walked down the hall to the New York branch offices of Chicago Investment Group.

He was always one of the first to arrive in the office, which had a staff of eight. When he walked in at 7.50 just one other colleague was already at work. Crossing over to the fridge he retrieved a breakfast of cottage cheese, fruit, and a glass of skimmed milk.

Mr Nadel settled down at his desk in the middle of the office and started going over commentaries on the state of the markets. Turning to his computer he called up several share price charts. Studying the trends, he looked for technical signs that the shares might be preparing to break out of their current trading ranges.

As he studied the screen, a small radio on his desk softly played light music. Looking out through the windows, he could see the rush hour traffic snaking up the West Side Highway to midtown Manhattan. To the West, across the Hudson River, lay the suburbs of New Jersey. At a height of around 1,000 feet above ground level, his view was completely unobscured by the surrounding buildings.

Around 8.30 his wife, Cheryl, called to wish him a good day at work. He replied that he loved her, and hung up. Of the many phone conversations they would have over the next few hours, it was the only one that could be described as routine.

It was almost 8:30am when Glenn Flood drove into the Pentagon's North Parking lot. The autumn weather was picture perfect. Many of the workers were jogging along the access roads or riding bicycles.

Most of the military employees had already arrived to do their "PT" - physical training. Civilians, like Mr Flood, say they get enough exercise just walking to and around the world's largest office building. Despite 17.5 miles of corridors, the physically fit can walk between any two points in the building in only seven minutes. And this, with no elevators to speed them on their way through 131 stairways.

The Pentagon is virtually a small city with about 23,000 employees who, an official bulletin boasts, "tell time by 4,200 clocks, drink from 691 water fountains, utilise 284 rest rooms and consume 4,500 cups of coffee each day".

The drab, aging Pentagon was built during the second world war along the Potomac River in Virginia on wasteland and swamps, about two miles from the White House. Not a great deal has been done since then to improve the landscape. It is flat, almost bare of trees, uninspiring - particularly in comparison with the imposing buildings, marble monuments and grand boulevards of the capitol.

But Mr Flood was not thinking of the terrain on Tuesday morning. He was preparing to write "talking points" for his boss, Donald Rumsfeld. The secretary of defence was making a push to resolve problems with the department's budget by cutting "waste and bureaucracy".

Every morning, Mr Flood - balding and bearded and who wears glasses "only to see" - scrolls down his computer screen, reading the daily compilation of defence stories from newspapers around the world. The television on his desk was tuned to CNN.

Written by:
Peter Thal Larsen
Richard Wolffe
Peter Spiegel
Victoria Griffith
Nancy Dunne
James Harding
Stephen Fidler
Farhan Bokhari
Gillian Tett
James Drummond
Mark Odell
Kevin Done
Abigail Rayner
Stephanie Kirchgaessner
Tally Goldstein

Part Two