The destruction of the twin towers was a site that became the definition of terror. It was an onslaught that killed nearly three thousand people - but a decade later, the attack isn't over. The dust that overwhelmed the streets of Lower Manhattan became a weapon. The clouds contained particles of pulverized concrete, asbestos, lead from fifty thousand computers, mercury from countless light bulbs -- a million tons of poisonous fallout. Now nearly 20,000 people are receiving treatment for a range of illnesses, many of them serious. And more than 60,000 are registered as at risk. The BBC Our World team first broke the story globally in 2006. They return 5 years later to investigate a worsening tragedy and followup with those who shared their stories.