The footage reveals the efforts of rescue teams making forays into small, undamaged voids, deep beneath the carnage above ground. What they found ranged from decaying bodies to, surprisingly, virtually untouched cars. While some places were completely destroyed, other areas – shopping malls and underground stations – were sufficiently intact for workers to walk around freely.

The documentary, to be shown on Channel 4 on 11 September, also focuses on the forensic teams' painstaking and often harrowing process of identifying victims. For some of those experts , the work became more than just a job.

Shiya Ribowsky, a former director of special projects at the New York City Medical Examiner's Office, found out that his best friend Jeff Wiener had been killed in the attack.

"Knowing that Jeff had been killed made it easier for me to work the kind of crazy hours we were working, because I felt as if I had a stake in this," he said. "It put a face for me on all the other victims, because it was easier for me to understand a family member or a friend sitting in front of me who was talking about 'this habit' of their friend or 'that attribute'. It made it easier for me to sympathise."

The amount of time that he and his team would have to remain at work was revealed in the first three weeks after 9/11 when only 290 bodies had been identified, with still no trace of the thousands of people reported missing.

Because of the efforts of the rescue workers and DNA comparison techniques used to identify the 20,000 body fragments, more than 60 per cent of the victims were finally identified and restored to their families.

http://news.independent.co.uk/world/americas/article2919642.ece