What caused the fire?

Following early speculation that there had been an "uncontained failure" of engine parts - effectively a jet compressor turbine shattering - the investigators moved on to looking at how the tyre blow-out could have led to the rupture of one of the fuel tanks in a high-velocity impact.

The US's National Transportation Safety Board (NTSB) released details of four incidents in which Air France Concorde's had blown tyres on take-off.

In each of the incidents which took place between 1979 and 1981, the NTSB said that tyre debris and wheel shrapnel damaged one of the engines, punctured wing fuel tanks and severed hydraulic control lines.

The board went on to recommend that each of Concorde's tyres and wheels should be carefully inspected before each take-off.

Unique design

Once the cause of an air accident has been established, air investigators move onto examining what role the design of the aircraft had in the incident.

Concorde's unique wing design

In Concorde's case, the investigators have been scrutinising the delta-wing design, unique in civil aviation.

Before this accident, Concorde was widely regarded as having been one of the most carefully designed aircraft in the world.

Safety testing of Concorde took years to complete as designers and engineers went to great lengths to make supersonic passenger travel a reality.

For instance, the twin-mounted engines positioned under each side of the delta wing are separated by a relatively thin but strong wall.

This is designed to protect a fully operational engine from damage caused to its neighbour.

Concorde was also designed to ensure that areas most likely to be affected by external damage such as the break-up of an engine are clear of essential mechanical parts.

The aircraft's three hydraulic command systems were also designed to operate independently of each other.

In theory this means that the aircraft can still be piloted even if one of the systems had been knocked out.

So why did it take off?

Given that the disaster began on the Paris runway, one of the key questions that the relatives of the dead have asked is why did Concorde AF4590 take off at all?

Clues: Investigators sealed off the crash site

Concorde is designed to operate safely in the event of the loss of an engine.

Former pilots say that Concorde is "very tough" and has an extremely large capacity to generate additional power when needed.

However, the pilot was aware of problems but had given the "V1" signal - the point beyond which take-off cannot be aborted.

Once in the air, he was attempting to stabilise the aircraft by increasing power and lift (the relationship between an aircraft's speed, wing design and angle of flight).

Had he done so, Mr Marty would have attempted to pilot on the fully-operational engines to the allotted emergency landing area at nearby Le Bourget Airport.

Pilots agree that this is the best approach to this kind of emergency.

Last-minute repairs

Shortly before take-off, pilot Christian Marty overruled an earlier decision not to replace part of a faulty thrust reverser on the number two motor, the engine under the port wing nearest the fuselage.

The repair is reported to have taken around half an hour. It was the same engine which Mr Marty then reported to be at fault following take off.

While this has formed part of the investigation, it is unclear whether this played any exacerbating role in the unfolding disaster.

Reconstruction

While establishing the facts may appear an almost impossible task, some of the world's best aviation experts and aviation engineers have studied Concorde for years.

Lockerbie: Reconstruction

They were helping air accident investigators within hours of the crash and one of the first priorities is to actually rebuild what remained of the aircraft.

It was this approach of painstakingly piecing together almost all of the Pan Am airliner blown up over Lockerbie, Scotland, in 1989 which proved that a bomb had been planted in a suitcase on board the aircraft.