The WTC steel was ordered to be recycled only days after 9/11, ensuring there couldn't be a worthwhile investigation into why the towers collapsed.
Many of the sites who talk about this like to emphasise the “speed” of the recycling operation. For example:
...the steel beams were quickly recycled before investigators even had the chance to look at them...
Sounds suspect, right? But it’s also untrue. First, the steel wasn’t entirely cleared from the site until May 2002, not perhaps as quick as the claims suggest.
May 29 2002: As the last steel column of the demolished World Trade Center was removed Tuesday, construction workers at the site were honored for their work there since September 11th.
Further, the recycling did not happen “before investigators even had the chance to look at them”. Here’s Dr W. Gene Corley, head of the Building Performance Assessment Team, in his testimony to the House of Representatives:
"There has been some concern expressed by others that the work of the team has been hampered because debris was removed from the site and has subsequently been processed for recycling. This is not the case. The team has had full access to the scrap yards and to the site and has been able to obtain numerous samples. At this point there is no indication that having access to each piece of steel from the World Trade Center would make a significant difference to understanding the performance of the structures".