It's just 5.45 am and hijackers Mohammed Atta and Abdulaziz al-Omari pass security at Portland airport in Maine, flying to Boston where they connect onto Los Angeles-bound American Airlines Flight 11.
During the next 90 minutes, 17 other hijackers stroll through security and board their flights, armed with knives or box cutters that none of the screening methods detect.
American Airlines Flight 11 takes off slightly late at 7.59 am, just as a conference opens in the Windows on the World bar at the top of the World Trade Center in New York.
Then in the next 20 minutes, United Airlines Flight 175 takes off, followed by American Airlines Flight 77 from Washington Dulles International Airport bound for Los Angeles. At 8.42 am, United Airlines Flight 93 departs Newark, following a lengthy delay.
While the four flying death traps begin their journeys, the outside world gets its first inkling of what is happening.
Already, at 8.19 am, two flight attendants on Flight 11 tell controllers they think they have been hijacked.
The FBI is alerted and at 8.37 am the military sends up fighter jets to track Flight 11. But too late: at 8.46 am, Flight 11 flies into the upper floors of the World Trade Center's North Tower.
As fire and smoke belch from the building, people in the second of the Twin Towers are instructed to stay put. Then at 9.02 am that advice is reversed and a public announcement says: 'You may wish to start an orderly evacuation.' Police and firefighter crews are already racing en masse to the bottom of Manhattan, little knowing how bad things will get.
One minute later, at 9.03 am, Flight 175 crashes into floors 77 to 85 of the South Tower.
President George W. Bush, visiting a Florida elementary school, has already been told about the first crash, which many assumed was an accident.
The president has sat down to read children The Pet Goat when the second plane hits. At 9.05 am, Mr Bush's chief of staff Andrew Card bends over and whispers in his right ear: 'America is under attack.' As thousands of first responders rush into the burning towers, and thousands of office workers stream out, vice-president Dick Cheney is evacuated from his normal White House office into a bunker.
A minute later, at 9.37 am, Flight 77 smashes into the Pentagon's west facade. 'At first I thought I've blown up the fax machine. Then I realised it wasn't me. I smell the jet fuel,' an accountant in the building recalls for the 9/11 memorial in New York.
At 9.42 am, all flights in US airspace are grounded and three minutes later the White House and Congress are evacuated.
Helpless people fling themselves from the upper floors of the World Trade Center as a horrified world looks on.
Many believe that the fourth hijacked plane, Flight 93, was intended to be flown into the capital. However on that flight, some passengers become aware through phone calls about the unfolding assault.
'Group some people and do the best you can to get control,' the mother of passenger Mark Bingham tells her son after delivering the terrifying news.
At 9.59 am the South Tower of the World Trade Center collapses, killing hundreds of workers who hadn't got out, as well as first responders who'd gone in.
At 10.03 am, Flight 93 crashes into a Pennsylvania field, apparently after passengers overpower the hijackers.
The North Tower collapses at 10.28 am, killing about 1,400 people within seconds. The disaster is caught by major television networks and broadcast live around to stunned viewers the world.
Over the next hours, a vast search and rescue effort begins in the apocalyptic scene in lower Manhattan. Amazingly, 14 people are found alive inside the rubble of the North Tower, where part of the stairwell shielded them from the collapse.
Dust, debris, fires and ruptured waterlines make getting the site under control difficult. Every few hours, rescuers find isolated survivors.
Mr Bush addresses the nation at 8.30 pm, declaring what he calls 'the war against terrorism.'
