A monument to them is almost complete, 17 years after the 9/11 attacks. Photo / Getty Images

It was 9.24am on September 11, 2001, when pilot Jason Dahl received a terrifying message: "Beware any cockpit intrusion. Two a/c (aircraft) hit World Trade Centre."

Four minutes later, hijackers struck, with the flight recorder picking up the sound of a struggle and shouts of, "Mayday!" and "Get out of here! Get out of here!"

The plane plunged 200m. In a chilling recording at 9.31am, al-Qaeda terrorist Ziad Jarrah is heard telling passengers in broken English: "Ladies and gentlemen. Here the captain, please sit down, keep remaining sitting. We have a bomb on board. So sit."

The other hijackers can be heard just outside the cockpit, ordering the co-pilots: "Lie down. Down, down, down."

A pilot answers: "Please, please, please … Please, please, don't hurt me. … Oh God."

The next words that can be distinguished are: "I don't want to die."

Nothing more is heard from the pilots of United Airlines Flight 93, reports news.com.au.

This was when one of the most remarkable and heroic stories of 9/11 unfolded, as brave passengers prevented an even worse tragedy befalling America on that fateful day, almost 17 years ago.

The last pieces of the plane's wreckage will be returned to the memorial at the spot where it crashed in Pennsylvania later this year — along with the final items collected from the debris.

It is a moment of recognition for the families of the 40 passengers and crew who died on the plane when it ploughed into a field just 20 minutes away from the terrorists' target — the Capitol building in Washington, DC.

A WINDOW OF OPPORTUNITY

Flight 93 was in a different position to the other three planes hijacked that day.

The aircraft had taken off late from a crowded Newark Airport in New Jersey and was running 25 minutes behind schedule on its way to San Francisco.

Only four terrorists were on the flight, as opposed to the five on the other planes. It's believed the fifth intended member of the jihadist gang had been stopped from entering the US at a Florida airport just weeks earlier in August.

Posted by Flight 93 National Memorial on Wednesday, 11 July 2018

While their co-conspirators had hijacked their planes within half an hour, these attackers waited 46 minutes before launching their assault.

It meant that when passengers began phoning loved ones to tell them the pilots had had their throats cut, they learnt the sickening news that the World Trade Centre had been hit by two planes.

Realising the gravity of their situation, a group of passengers decided their only hope was to storm the cockpit — and began to devise a plan.

Among the courageous group was Todd Beamer, who had left his pregnant wife Lisa behind in New Jersey; Mark Bingham from California, a man later honoured as "smashing the gay stereotype"; Jeremy Glick, whose daughter Emerson was just three months old; and Tom Burnett, the first passenger to get a call through to someone — his flight attendant wife Deena.

He told her the plane had been hijacked by people claiming to have bomb, but that he didn't believe they did. A passenger was stabbed, he added. In a second call, she told him about the Twin Towers, and he said the hijackers were "talking about crashing this plane … Oh my God. It's a suicide mission".

His final words were: "Don't worry, we're going to do something."

Mr Beamer could only reach an airline phone operator. He asked her to tell his family he loved them, and then addressed the others with the now famous words: "Are you ready? Let's roll."

The group armed themselves with cutlery, boiling water, fire extinguishers and drink carts and began moving quickly towards the front of the plane.

THE FIGHTBACK

In the next recordings from the cockpit — revealed in the 9/11 Commission report released 14 years ago this weekend — the passengers can be heard fighting to enter the cockpit.

Loud thumps, crashes and shouts are heard as Jarrah begins to tip the plane sharply from left to right and forward and back.

"Roll it!" Mr Burnett is heard shouting, most likely in reference to a drinks cart they were using to ram the door.

"In the cockpit!" screams someone else. "If we don't, we'll die!"

At 10.01am, Jarrah shouts, "Allah is the greatest!" and asks another hijacker in the cockpit, "Is that it? I mean, shall we put it down?"

His fellow attacker says yes, with the passengers apparently about to breach the cockpit. "Pull it down! Pull it down!" he yells.

With the sound of the passengers still attacking, the plane ploughs into an empty field in Shanksville, Pennsylvania, at 933km/h, about 20 minutes' flying time from Washington, DC.

United 93 was the only flight not to reach its intended target, thought to be the Capitol building in the nation's heart, where Congress was in session. The passengers' daring may have saved hundreds of lives.

'I JUST WANT TO SAY I LOVE YOU AND I'LL MISS YOU'

Last week, the final piece of the Tower of Voices was put in place at the Flight 93 National Memorial in that remote Pennsylvania field.

The last stage of construction will involve hanging 40 individually tuned chimes inside the Tower, representing each of the 33 passengers and seven crew members who died on the plane.

The monument, which will be one of the world's largest musical instruments, will be dedicated on September 9, almost two decades after the terror attack.

It has been a long wait for the families of those who died on Flight 93, with the memorial plaza completed in 2011, 10 years after the attacks.

Then-president George W. Bush and his predecessor Bill Clinton stepped in to speed up the process of completing the memorial to the passengers and crew who "changed the course of American history".

Gordon Felt, whose brother Edward was killed that day, told the Washington Post in 2015: "Our loved ones left a legacy for all of us. If the Capitol building was destroyed that day, just think how much more devastating an impact that day would have had on our country.

"They could have sat back and let others dictate the end of their lives. But they fought back and became heroes in the process."

Interior Secretary Sally Jewell called the people on Flight 93 "modern-day heroes who represent the very best in us".

She quoted from Abraham Lincoln's Gettysburg Address: "We have come to dedicate a portion of that field as a final resting place for those who here gave their lives that that nation might live."

In one of the many heartbreaking voicemail recordings that can be heard at the Flight 93 National Memorial Visitor's Centre, Lauren Grandcolas tells her husband: "We're having a little problem on the plane. I'm totally fine. I just love you more than anything, just know that."

In a preternaturally calm message for her sister, Linda Gonlund says: "Elsa, it's Lin. I'm on United 93. It's been hijacked by terrorists who say they have a bomb.

"Mostly I just want to say I love you and I'll miss you.

"I don't know if I'm going to get the chance to tell you that again or not."

— The 9/11 Commission report is the official report of the events leading up to the September 11 terrorist attacks. It was released 14 years ago today. To read the full report, click here.