Who and what caused them to do the attack of the September eleven?

Answer 1

Dr. Tawfik Hamid, an Islamic scholar who works toward spreading a peaceful version of Islam, explains in his books The Roots of Jihad and Inside Jihad that the attack of Sept 11, 2001 was motivated by the Islamic principle of Jihad. Jihad is interpreted by extremist Muslims that they must do whatever is necessary to establish the dominance of Islam around the world.

Dr. Hamid in his younger years as a medical student in Egypt was lured into and, for a substantial period of time, participated in a terrorist group in Cairo. He was an associate there of Zawahiri, now the head of Al Qaeda, but left the group when they wanted him to begin to commit violent acts.

As Dr. Hamid writes: Islamic terrorists search for the most crowded areas, and time their action to kill and inflict harm and pain to the maximum number of those whom the Muslims call "infidels" so they can please Allah and inherit the paradise (from the Preface in The Roots of Jihad).

This same concept of Jihad was the motivation/justification for the recent 2013 Boston Marathon bombings.

Jihad also has been and continues to be the motivation for suicide bombings in Iraq and Afghanistan that kill both local residents and American soldiers. It was the motivation for the Ft. Hood killer Nidal Hasan who shot American soldiers at a military base in the US in 2009, and even for Malvo and Muhammad, the DC Beltway Snipers of 2002.

More moderate Muslims regard Jihad as inner struggle to become a better person. Islamic extremists who commit terrorist acts however genuinely believe that they are serving religious goals with Jihadist acts.