Where they once stood proudly, now there is empty space. They were the symbols of American financial might, designed to meet the busy demands of the 21st century. In the event, the twin towers lasted only 21 months of the century they were supposed to dominate. Today, Ground Zero is a poignant place. Makeshift memorials mark the spot where so many people lost their lives a year ago today. America has learned to live with the disaster which still casts a shadow over every single day in New York. But it, and the world, will never forget... STEVE CARROLL retraces the events of September 11

NO ONE noticed him take his seat. On board American Airlines Flight 11, no one paid any attention as Mohamed Atta pulled out his mobile phone and made a call.

Across the runway, inside United Airlines Flight 175, Marwan Al-Shehhi answered immediately.

"The plot is go" Atta told him and hung up.

Not a single passenger noticed as Atta and Al-Shehhi calmly set off a chain of events that plunged an entire nation into crisis.

Hundreds of miles away, workers were filing out of the subway system and squinting at the brilliant, sunny New York morning.

Many were already inside the North Tower of the World Trade Centre, beginning another day at the office. But in the skies their fates had already been predetermined.

On Flight 11, Atta delved into his small bag and pulled out a home-made flick-knife. At his nod, four conspirators did the same.

Jumping from his seat, he lurched forward into the cockpit. He told the pilots he had a bomb and took control of the aircraft heading towards Los Angeles.

Then he changed course.

Inside the North Tower, few heard Flight 11 glide effortlessly over Manhattan. But at 8.46am, everyone heard the bang.

Everyone saw the fireball that engulfed the skyscraper. And, in the streets below, everyone started running.

The jet, laden with thousands of gallons of kerosene, had caused carnage. It struck the tower at floors 101 to 105 and hundreds died immediately.

Above those floors, office workers who escaped immediate injury desperately tried to reach the escalator and stairways. Their paths were blocked.

Their last thoughts were for their loved ones, those who were going to be left behind.

As they stared death in the face, they rang home.

"Sean, it's me" sobbed Melissa Hughes. "I'm stuck in this building in New York. A plane hit the building or a bomb went off we don't know, but there's lots of smoke and I just wanted you to know that I love you. Bye bye."

They smashed windows desperately trying to get some air. The smoke and the heat was overpowering. Many knew they were doomed.

From below, stunned people - their eyes transfixed skywards - noticed new objects falling from the tower. It looked like debris. But then the ground, the awful truth dawned.

They were people.

In the South Tower, hundreds of people were already running. They had seen the plane hit. They had seen the fireball.

But others were staying. Voices had crackled over the public address system. The tower was secure. It was unsafe to leave.

Just sixteen-and-a-half minutes after the first impact, Flight 175 hurtled over the Hudson River, dipped its wing and smashed into the second tower. Taking out floors 78 to 84, many died instantly. Many who had believed they were safe.

Millions watched the second impact live on television. And as they watched, straining their eyes to take in what they were seeing, American Airlines Flight 77 struck the Pentagon, in Washington.

Motorists lodged on Washington Boulevard heard the screaming of the engine. Their vehicles shook as the plane wobbled barely 50 feet overhead and fell to earth, destroying the south-west wall.

America convulsed. And worse news followed. A fourth plane had been hijacked. Yet it was the events on board United Airlines Flight 93 which gave a nation the strength to fight back.

A passenger on that plane, Tom Burnett, called his wife. Deena was watching the outrages unfold on television from her home in California.

He told her the plane had been hijacked. He pleaded with her to call the authorities. "I know we're all going to die," he said. "There's three of us who are going to do something about it."

The flight from Newark to San Francisco had turned around. But no one on board knew where it was going. And some now knew the twin towers had been hit.

"I know I'm not going to get out of this," Todd Beamer told airphone operator Lisa Jefferson. "We're going to do something."

He dropped the phone.

"Are you guys ready?" she heard him say. "Let's roll."

It is not known what happened next. But the aircraft banked sharply and finally hit the ground, scattering debris as it ploughed into a field near Shanksville, Pennsylvania.

Back in New York, panic had set in. Inside the twin towers, stairways were blocked as the frantic race to escape took over.

But as thousands of workers got out, hundreds of emergency service workers were still storming in.

Racing up floor after floor, these heroes battled vainly to aid the escape, against a fire they could not hope to control.

They had little time. The intense heat was melting the core of the twin towers buildings. Their unique design was also their weakness.

The designers had not discounted the possibility that it could be hit, they simply had not envisaged such a devastating terrorist attack.

Only 56 minutes after it was struck, the South Tower creaked, bent and fell to earth sending a massive pall of smoke, dust and debris through the city's financial district.

New York turned grey in seconds. Twenty minutes later, the North Tower suffered the same fate, its trademark antenna surrendering and disappearing into the ground.

For those still inside there was no escape.

In only 102 minutes, 3,027 people died. Many were killed because of the job they were doing or where they were. All were victims of the world's worst terrorist atrocity.