Jones was not the first to propose this possibility. The first people were firemen who had to deal with molten hot fires for 12 weeks at ground zero. Some steel beams would burst into flames when exposed to air 10 weeks later. Jones made headlines in 2005 when he argued that the rapid and symmetrical fall of the World Trade Center looked like the result of pre-positioned explosives. He argued that fires alone wouldn't have been hot enough to crumble the buildings; and that even if struck by planes, the towers should have been strong enough to support the weight of the tops as they crumbled — unless they were leveled by explosives.

Essentially forced to retire, Jones says he is now paying for research out of his own pocket. He likens himself to Galileo and Newton, who stood by their consciences. "I would like to think I could stand up for the truth,".

The dust study vindicates his earlier theories, Jones says, but he has mixed feelings about the implications.

Scientists that look at the evidence now are sadly seldom American.
http://www.esoterictube.com/danish-chemist-found-nano-thermite-in-wtc-dust.html

Given the 2 weeks of nightly work in the WTC elevator shafts 3 weeks prior to 9-11 there is opportunity. Motive is crazy but there have been stranger things, so what we have from my pov is means and opportunity.

what and how is largely answered. Why is too painful to debate.