When did they have a chance to put the explosives in the WTC Towers?

People may wonder when was there a chance to place explosives in the World Trade Center towers. Here is a brief video that helps to explain this. Starting in about a minute and a half into the video there is a discussion about peopple who worked in the World Trade Center Towers who said that prior to 9/11 there had been several unexplained evacuations and power downs in the towers.

9/11 Truth: Unusual Evacuations & Power-Downs in the WTC Prior To 9/11



Now that I think about it, there were many parts of the WTC that were vacant (not rented) before 9/11, this probably shortened the amount of time needed for the buildings to be evacuated. Here is an article (see second item called "Skyscaper Award") that talks about how before 9/11 the WTC 7 was mostly vacant and after 9/11, when it was rebuilt, it has become attractive as office space.
http://www.downtownexpress.com/de_211/undercover.html