What happened on United 93 flight on 9/11?
Heroic passengers who died while foiling a terrorist hijacking of a passenger plane remembered 25 years on
Memorials are being held in New York and beyond in memory of the 9/11 terrorist attacks, 25 years on.
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At a memorial ceremony in New York City, the first of six moments of silence was held this morning, with US president Donald Trump in attendance.
A large crowd gathered solemnly just outside barricades surrounding the 9/11 Memorial, and in a nearby park, where hundreds stood in silence.
Others are paying tribute at a memorial at a field in Pennsylvania, where the United Airlines Flight 93 crash-landed amid the September 11 attacks - with passengers fighting back against the hijackers.
This is what happened.
What happened on United 93?
At 8.46am on 11 September 2001, 2,977 people were killed when hijackers crashed into the World Trade Center in New York City and the Pentagon in Virginia.
A fourth plane missed its target after passengers learned of the other hijackings, with those on board bravely attempting to overpower the terrorists, ultimately crashing the aircraft into a field in Pennsylvania.
It was the final plane to take off and the only flight taken over by extremists that did not reach its intended target.
United 93 was due to fly from Newark in New Jersey to San Francisco in California.
It was delayed by 42 minutes, and the four hijackers (Ziad Jarrah, Ahmed al-Haznawi, Ahmed al-Nami, and Saeed al-Ghamdi) waited 46 minutes before starting their action.
The al-Qaeda-aligned terrorists killed one passenger and stormed the cockpit of the plane. Jarrah, who was a trained pilot, took over and diverted the plane back to the east coast.
Due to the delay, passengers had begun to hear about the other hijackings and how flights had crashed into buildings.
They then fought back against the hijackers, and Jarrah, fearing he might lose control, crash-landed in Shanksville in Somerset County, Pennsylvania.
How many died on United 93?
Everybody on board United 93 died: seven crew members, the four hijackers, and 33 passengers.
There was intended to be a fifth hijacker, Mohammed al-Qahtani, but he did not make the trip as he had been intercepted for his reportedly dubious immigration status.
Among the passengers were Todd Beamer, Mark Bingham, Tom Burnett, and Jeremy Glick, who formed a plan to retake the plane. They voted to try and sabotage the hijacking when they realised what was intended.
Beamer's final words, heard on an aeroplane phone, were: "Are you guys ready? Okay. Let's roll!"
Jarrah responded to the flight by pointing the plane to the sky, but the passengers persisted, and the crash landing occurred only a few minutes after the fightback began.
'Let's roll' later became a national catchphrase and was used in speeches by George W Bush.
What was the most likely target of Flight 93?
The United States Capitol building is understood to have been the target.
The building on Capitol Hill in Washington, DC, hosts the legislative branch of the federal government.
Was there a pregnant woman on Flight 93?
Yes, Lauren Grandcolas, 38, was three months pregnant with her first child.
A phone call she made on board the aeroplane to her husband, Jack Grandcolas, was recorded: "Jack, pick up, sweetie, can you hear me? Okay. I just want to tell you, there's a little problem with the plane. I'm fine. I'm totally fine. I just want to tell you how much I love you."
Did anyone survive United 93?
Nobody survived the plane crash. Stoney Creek resident Nevin Lambert is thought to be the only witness to the actual crash and said he saw United 93 upside down in a 45-degree-angled nosedive.
By acting, the passengers saved potentially hundreds, if not thousands, more lives and are being remembered as part of the 9/11 tragedy.
The Flight 93 National Memorial now stands at the site.