bringing down the towers
It is best not to get too distracted with how and when the towers were rigged, as the "why" is much more important (part 1.5).
For those intrigued by the subject, we can get you started:
> Twin Towers Security run by Marvin Bush
The day after Bush took office in 2001, he handed his brother a $50 million+ contract for his company Securacom (now known as Stratesec) to handle security for both the Twin Towers and Dulles Airport - from supplying armed vehicles and guards to installation of surveillance equipment. He had absolutely no background to indicate he could handle such a contract or the job it entailed, but this is precisely how every Bush got rich - with government contracts given to them (usually by other Bush family members - dating back earlier than Prescott Bush). Marvin was in charge of security on and before 9/11 at both the WTC and Dulles Airport (origin for flight 77 into the Pentagon). On the weekend of 9/8 and 9/9 there was a 'power down' condition in WTC 2, the south tower. This blackout condition meant there was no electrical supply for approximately 36 hrs - from floor 50 upwards. Without power there were no security cameras, no security locks on doors, and reportedly many, many 'engineers' coming in and out of the tower. As one can imagine, infiltration possibilities are endless and easy with full control over security of both towers. As coincidence would have it, Marvin Bush quickly jumped ship from Securacom - only to immediately join the board of HCC insurance, one of the major policy carriers for the WTC...the Bush family sure knows how to cash in. [source 1] [source 2] [source 3]
> Mysterious foreign artist groups with WTC Construction Passes
Artist groups known as "E-TEAM" and GELATIN (later changed to Gelitin) were selected to be part of the Lower Manhattan Cultural Council (LMCC) World Views / StudioScape programs that allowed them and other artists to work (and live) in the WTC throughout the four years leading up to 9/11 on different floors, including 90, 91, and 92 of the North Tower. Additionally, they were given 24/7 construction access to the WTC that allowed them to freely move all sorts of materials in and out of the complex for their art project, "The 'B' Thing" - which involved creating a very temporary "balcony" off the 91st floor of WTC1 [NY Times article]
During this project almost the entire 91st floor of WTC1 was filled with "empty boxes" all labeled "BB 88"    which just so happens to correspond with a specific and interesting product (Littelfuse electrical 3-phase fuse holders). The BB 88 parts may or may not have something to do directly with the demolition - one thing is for sure, it is highly unusual for art students to have such a massive amount of them. More likely, they could simply be using BB 88 boxes with completely different contents inside. Shortly after this in March 2001, another art group known as the E-TEAM (Explosive Team?) lights up the WTC in the same area with their art group name (E-TEAM) using 127 illuminated windows, which coincidentally just so happen to be in the exact location where the plane impact would take place one month later.
Just prior to 9/11, GELATIN released a disturbing and poorly drawn art book about their project "The B-Thing", which contains some odd and in-your-face imagery, including a drawing of someone falling from the towers with the caption "300 meters of pure pleasure"; an image of the tower being destroyed with the caption "Depression inside, amazement outside", and even a picture of the towers with a line above them and the caption "Last chance to open a parachute". The book itself is $600+ on Amazon.com, going as far as to describe the project as "an unbelievable, completely illegal, and fully secret stunt when it was performed... "The B-Thing" is now unbearably surreal, weirdly prescient, and forever unrepeatable.".
["The B-Thing" from Gelatin's official website] [Project Art Gallery] [MarkDotzler] [TheRunDownLive]