US did it, specifically the Pentagon. How?
You ask, US went and invaded sovereign countries, without any reason, just because you want free oil and resources, and what do you think?
You think the Muslims will just stand by and watch you loot their homeland? Keep on dreaming!
Terrorism is a very recent issue, born from mother greed and uncle Sam. The Muslims can't invade US and defeat it, but they want revenge, so 9/11 is a direct consequence of your own country's incompetent decisions, it's not an accident, or terrorism.
Some might call it justice, but it is indeed retribution for your actions, NOTHING goes unpunished, it always comes back to bite you, sooner or later.