I think most of the conspiracy claims are nonsense, based on people with no idea about the fields of knowledge they are discussing pontificating on what is and is not possible in aviation, demolitions, physics, chemistry etc etc.

Most of their accusations are easily rebuffed imo.

However, going on past history, the Gulf of Tonkin incident, Pearl Harbour, WMD's in Iraq, the planned self attck on Americans to provoke a war with Cuba etc etc I think it's a high probablity that the US government, or at least various parts of it knew something was going to happen and did little to prevent it, as anything that would happen could be a useful pretext for a retaliatory war.