You quizzed Panorama reporter Jane Corbin on what drove the hijackers to kill thousands of civilians, and themselves, in a live forum.

To watch an on-demand version of this forum click here:

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September 11th was the most devastating terrorist attack the world has ever seen.

Panorama has followed the trail the hijackers left across three continents.

The programme reveals the planning, the tradecraft and the financing of a secret terror cell in Europe and explains why the cell went undetected.

This is the inside story of 19 men whose terrorist atrocities changed the world forever.

Why weren't the hijackers detected before the attacks? What motivated them? How much of a trail did the terrorists leave?