'They pulled out knives and said they were taking over'

It was just before 9 am, and the scene on board United Airlines flight 93 from Newark to San Francisco was teetering on the brink of hysteria. One man had locked himself in the toilet, begging police to believe him as he talked on his cellphone that the plane had been hijacked.

Another, Jeremy Glick, was on the line to his wife Liz, telling her there were three Arab men on board with knives and "a large red box that they claimed contained a bomb".

And then something extraordinary happened. Mr Glick and a handful of fellow passengers realised they were beyond salvation, but decided they were going to act to ensure as few others as possible would have to share their fate.

Mr Glick told his wife he and his comrades-in-arms thought they had nothing to lose by "rushing" the hijackers. Another passenger, Thomas Burnett, of San Ramon, California, told the same thing to his wife in a cellphone conversation patched through to the police. "I know we're all going to die," he said, in an account of the call relayed to reporters by the family pastor. "There's three of us who are going to do something about it." Mr Burnett's last words to his wife were: "I love you honey." The exact sequence of events is not known exactly – one report suggested there was some kind of explosion and a plume of white smoke in the cabin before contact was lost – but the result was that flight 93 crashed into the ground in Pennsylvania near an airstrip 80 miles southeast of Pittsburgh. All 45 people on board perished, but nobody was injured on the ground. According to James Moran, the Virginia Congressman, the hijackers had intended to fly the plane another 85 miles into the presidential retreat at Camp David.

Yesterday, some of the horror of what transpired on the four hijacked aircraft came to light as investigators took their first look at passenger lists, air traffic control transcripts and the testimony of passengers talking on cellphones.

The clearest lesson to emerge was that, for all the meticulous planning that went into the attacks on the World Trade Centre and the Pentagon, the core concept was devastating in its simplicity.

The men who hijacked the four airliners were not armed with any fancy weaponry. They did not need to build any kind of explosive device or smuggle hi-tech equipment across international borders and airport security systems.

Instead, their only weapons were a few innocuous-looking knives – nothing that could not be taken on board in a wash bag, according to the result of the initial investigation – and their own fanaticism. The "bombs" that they used to bring death to thousands of people were the planes themselves, all four loaded with as much as 24,000 gallons of jet fuel at the start of what were supposed to be routine flights across the American continent.

The planes became many orders of magnitude more powerful than the truck bomb used to blow up the federal building in Oklahoma City. And they brought death and destruction on a scale that is unprecedented outside the worst aerial bombardments of wartime.

The first reconstructions suggested the hijackers worked in groups of three to five per plane. In each case, they waited until the plane had completed its take-off and was approaching cruising altitude.

They then pulled out their weapons – "knife-like" objects that could have been razor blades or short box-cutters – announced they were taking over the plane and herded the passengers as far away from the cockpit as possible. On at least two others, they stabbed and killed flight attendants, apparently to create a diversion and get the pilot out of the cockpit. One theory suggested they actually killed the pilots to ensure that they would be left unchallenged at the commands.

They then announced to air traffic control that they were in charge, requested flight paths back to their target cities and closed off the transponders to ensure their subsequent movements were as difficult to track as possible. Clearly, each team included at least one trained pilot with enough experience to fly a commercial jumbo manually – in most, if not all cases, the automatic pilot was also switched off.

The first plane to be taken over was American Airlines Flight 11, which took off from Boston's Logan airport bound for Los Angeles and slammed into the north tower of the World Trade Centre. Air traffic controllers in the Boston area heard a hijacker give instructions to the pilot in English.

"He was saying something like 'Don't do anything foolish. You're not going to get hurt'," a controller told the Christian Science Monitor. Someone – either the pilot or the hijacker – then made a request for a flight path to JFK airport in New York. The plane made an abrupt turn at 29,000ft above Albany, in upstate New York. One last statement before communication was lost was not fully understood at the time but soon became chillingly clear: "We have more planes, we have other planes."

Meanwhile, the scene in the passenger area was turning to mayhem. On AA flight 11 and possibly on the other plane out of Boston, United Airlines flight 175, the hijackers pulled weapons out of carry-on luggage and began stabbing the cabin crew.

"They started killing stewardesses in the back of the plane as a diversion. The pilot came back to help and that is how they got into the cockpit,'' one investigative source told The Boston Herald. "People were calling from the plane saying they were getting killed, calling 911,'' the paper reported. Reports of the cellphone calls from all four planes have been almost ghoulish: the flight attendant and mother of three en route to San Francisco who called her husband in terror to report that a colleague had been stabbed; the unidentified man who locked himself in the toilet on Flight 93; and Barbara Olson, the television commentator and wife of one of America's most powerful lawyers, telling her husband from AA 77, the flight that crashed into the Pentagon, that the passengers were being herded to the back of the plane and wondering what she should do.

Quite how the hijackers planned their assault and assembled to carry it out in the early hours of Tuesday morning is likely to be a big part of the official investigation. First reports suggested some of the men flew in to Boston that morning from Maine, having crossed the border from Canada. At least one rental car used by one or more of the men was recovered from a car park at Logan airport.

Getting through airport security may have been easier than anyone would suspect. According to Federal Aviation Authority officials, knives with blades less than two inches long are permitted as long as they do not seem to have a hostile intent. It probably did not help that the security checkers were minimum-wage workers with little incentive or training.

According to the investigative sources contacted by the Boston Herald, the weapons may have been assembled on board from razor blades and plastic holders.

So simple. So hard to detect. So devastating in its consequences. What is perhaps most shocking is not how exceptional it was, but how easy it might be to do the same thing again.