By Richard Lister
BBC News, Washington
A makeshift memorial for Flight 93 was created within days
Just after ten o'clock in the morning on 11 September, 2001 people living in the rural communities of Shanksville, Indian Lake and Lambertsville Pennsylvania, heard the thump of an explosion, loud enough to rattle windows.
Looking to where the sound came from, they saw an enormous ball of black smoke and flame, rising over the rolling open fields and woodland where United Flight 93 had just hurtled straight into the ground.
So great was the impact that the aircraft, a Boeing 757 with 44 people on board, completely disintegrated.
Debris was scattered over a radius of several kilometres, but there were few pieces more than a metre in size, according to those who got to the site first.
The plane was just 20 minutes flying time from Washington DC and American officials now believe its intended target was probably the White House.
An even greater disaster was prevented because the passengers decided to fight back.
When the four hijackers took over, passengers used mobile phones to raise the alarm and call loved ones.
They also planned to take on the hijackers themselves.
One, Todd Beamer from New Jersey was heard to say: "Are you guys ready? Let's roll!"
Drawing the memories and transcripts of those calls together the 9/11 Commission concluded that it was the passengers' efforts to regain control of the plane, that forced the four hijackers to cut their mission short.
The heroism on board Flight 93 has become an iconic part of the whole terrible story of that day.
It has been the basis for a major film, books and internet memorials. If you drive into the open country near Somerset in south-western Pennsylvania, a cobweb of roads eventually brings you to a makeshift temporary memorial a few hundred metres from the crash site.
There are a row of flags and a section of tall metal fencing bedecked with flowers, toys, flags and plaques left by well-wishers.
Passengers like Todd are credited with saving lives on the ground
It has the temporary feel of something heart-felt, but erected hastily in the immediate aftermath of the event. Building a permanent memorial at the site has proved problematic.
Congress authorised the National Park Service (NPS) to go ahead with a memorial in March 2002 and teamed up with a group representing victims' families to work with the network of people who own the land involved.
Deals were done to acquire about 80% of the land required, but negotiations with seven landowners proved unsuccessful.
So now the Park Service says it is resorting to an unusual move to force the landowners to sell. It has announced plans to condemn the land and have a judge fix a sale price for the NPS to buy it.
The landowners range from a quarry company - Svonavec - to a man whose grandfather bought several hectares in the 1930s. Another family operates a scrap metal company nearby, which would have to be moved.
They are unhappy about the announcement and some say the park service had not entered into formal negotiations with them before the announcement was made.
It will take some time for the legal process to be completed, but it appears that this move by the NPS will now bring this awkward chapter in the aftermath of 9/11 to a close.
Plans for the memorial on the Park Service website show a wide open space, covering almost 900 hectares.
The flight path will be paved in black granite, the names of those killed by the hijackers carved into 40 polished white granite slabs, lining a walkway.
Beyond a bronze ceremonial gate, the impact site will be planted with wildflowers.